Merge docs on consent bits from PR #3268 into release branch

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+Support in Synapse for tracking agreement to server terms and conditions
+========================================================================
+
+Synapse 0.30 introduces support for tracking whether users have agreed to the
+terms and conditions set by the administrator of a server - and blocking access
+to the server until they have.
+
+There are several parts to this functionality; each requires some specific
+configuration in `homeserver.yaml` to be enabled.
+
+Note that various parts of the configuation and this document refer to the
+"privacy policy": agreement with a privacy policy is one particular use of this
+feature, but of course adminstrators can specify other terms and conditions
+unrelated to "privacy" per se.
+
+Collecting policy agreement from a user
+---------------------------------------
+
+Synapse can be configured to serve the user a simple policy form with an
+"accept" button. Clicking "Accept" records the user's acceptance in the
+database and shows a success page.
+
+To enable this, first create templates for the policy and success pages.
+These should be stored on the local filesystem.
+
+These templates use the [Jinja2](http://jinja.pocoo.org) templating language,
+and [docs/privacy_policy_templates](privacy_policy_templates) gives
+examples of the sort of thing that can be done.
+
+Note that the templates must be stored under a name giving the language of the
+template - currently this must always be `en` (for "English");
+internationalisation support is intended for the future.
+
+The template for the policy itself should be versioned and named according to 
+the version: for example `1.0.html`. The version of the policy which the user
+has agreed to is stored in the database.
+
+Once the templates are in place, make the following changes to `homeserver.yaml`:
+
+ 1. Add a `user_consent` section, which should look like:
+
+    ```yaml
+    user_consent:
+      template_dir: privacy_policy_templates
+      version: 1.0
+    ```
+
+    `template_dir` points to the directory containing the policy
+    templates. `version` defines the version of the policy which will be served
+    to the user. In the example above, Synapse will serve
+    `privacy_policy_templates/en/1.0.html`.
+
+
+ 2. Add a `form_secret` setting at the top level:
+
+
+    ```yaml
+    form_secret: "<unique secret>"
+    ```
+
+    This should be set to an arbitrary secret string (try `pwgen -y 30` to
+    generate suitable secrets).
+
+    More on what this is used for below.
+
+ 3. Add `consent` wherever the `client` resource is currently enabled in the
+    `listeners` configuration. For example:
+
+    ```yaml
+    listeners:
+      - port: 8008
+        resources:
+          - names:
+            - client
+            - consent
+    ```
+
+
+Finally, ensure that `jinja2` is installed. If you are using a virtualenv, this
+should be a matter of `pip install Jinja2`. On debian, try `apt-get install
+python-jinja2`.
+
+Once this is complete, and the server has been restarted, try visiting
+`https://<server>/_matrix/consent`. If correctly configured, this should give
+an error "Missing string query parameter 'u'". It is now possible to manually
+construct URIs where users can give their consent.
+
+### Constructing the consent URI
+
+It may be useful to manually construct the "consent URI" for a given user - for
+instance, in order to send them an email asking them to consent. To do this,
+take the base `https://<server>/_matrix/consent` URL and add the following
+query parameters:
+
+ * `u`: the user id of the user. This can either be a full MXID
+   (`@user:server.com`) or just the localpart (`user`).
+
+ * `h`: hex-encoded HMAC-SHA256 of `u` using the `form_secret` as a key. It is
+   possible to calculate this on the commandline with something like:
+
+   ```bash
+   echo -n '<user>' | openssl sha256 -hmac '<form_secret>'
+   ```
+
+   This should result in a URI which looks something like:
+   `https://<server>/_matrix/consent?u=<user>&h=68a152465a4d...`.
+
+
+Sending users a server notice asking them to agree to the policy
+----------------------------------------------------------------
+
+It is possible to configure Synapse to send a [server
+notice](server_notices.md) to anybody who has not yet agreed to the current
+version of the policy. To do so:
+
+ * ensure that the consent resource is configured, as in the previous section
+
+ * ensure that server notices are configured, as in [server_notices.md](server_notices.md).
+
+ * Add `server_notice_content` under `user_consent` in `homeserver.yaml`. For
+   example:
+
+   ```yaml
+   user_consent:
+     server_notice_content:
+       msgtype: m.text
+       body: >-
+         Please give your consent to the privacy policy at %(consent_uri)s.
+   ```
+
+   Synapse automatically replaces the placeholder `%(consent_uri)s` with the
+   consent uri for that user.
+
+ * ensure that `public_baseurl` is set in `homeserver.yaml`, and gives the base
+   URI that clients use to connect to the server. (It is used to construct
+   `consent_uri` in the server notice.)
+
+
+Blocking users from using the server until they agree to the policy
+-------------------------------------------------------------------
+
+Synapse can be configured to block any attempts to join rooms or send messages
+until the user has given their agreement to the policy. (Joining the server
+notices room is exempted from this).
+
+To enable this, add `block_events_error` under `user_consent`. For example:
+
+```yaml
+user_consent:
+  block_events_error: >-
+    You can't send any messages until you consent to the privacy policy at
+    %(consent_uri)s.
+```
+
+Synapse automatically replaces the placeholder `%(consent_uri)s` with the
+consent uri for that user.
+
+ensure that `public_baseurl` is set in `homeserver.yaml`, and gives the base
+URI that clients use to connect to the server. (It is used to construct
+`consent_uri` in the error.)