initial WIP of a tentative preview_url endpoint - incomplete, untested, experimental, etc. just putting it here for safekeeping for now

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+URL Previews
+============
+
+Design notes on a URL previewing service for Matrix:
+
+Options are:
+
+ 1. Have an AS which listens for URLs, downloads them, and inserts an event that describes their metadata.
+   * Pros:
+     * Decouples the implementation entirely from Synapse.
+     * Uses existing Matrix events & content repo to store the metadata.
+   * Cons:
+     * Which AS should provide this service for a room, and why should you trust it?
+     * Doesn't work well with E2E; you'd have to cut the AS into every room
+     * the AS would end up subscribing to every room anyway.
+
+ 2. Have a generic preview API (nothing to do with Matrix) that provides a previewing service:
+   * Pros:
+     * Simple and flexible; can be used by any clients at any point
+   * Cons:
+     * If each HS provides one of these independently, all the HSes in a room may needlessly DoS the target URI
+     * We need somewhere to store the URL metadata rather than just using Matrix itself
+     * We can't piggyback on matrix to distribute the metadata between HSes.
+
+ 3. Make the synapse of the sending user responsible for spidering the URL and inserting an event asynchronously which describes the metadata.
+   * Pros:
+     * Works transparently for all clients
+     * Piggy-backs nicely on using Matrix for distributing the metadata.
+     * No confusion as to which AS
+   * Cons:
+     * Doesn't work with E2E
+     * We might want to decouple the implementation of the spider from the HS, given spider behaviour can be quite complicated and evolve much more rapidly than the HS.  It's more like a bot than a core part of the server.
+
+ 4. Make the sending client use the preview API and insert the event itself when successful.
+   * Pros:
+      * Works well with E2E
+      * No custom server functionality
+      * Lets the client customise the preview that they send (like on FB)
+   * Cons:
+      * Entirely specific to the sending client, whereas it'd be nice if /any/ URL was correctly previewed if clients support it.
+
+ 5. Have the option of specifying a shared (centralised) previewing service used by a room, to avoid all the different HSes in the room DoSing the target.
+
+Best solution is probably a combination of both 2 and 4.
+ * Sending clients do their best to create and send a preview at the point of sending the message, perhaps delaying the message until the preview is computed?  (This also lets the user validate the preview before sending)
+ * Receiving clients have the option of going and creating their own preview if one doesn't arrive soon enough (or if the original sender didn't create one)
+
+This is a bit magical though in that the preview could come from two entirely different sources - the sending HS or your local one.  However, this can always be exposed to users: "Generate your own URL previews if none are available?"
+
+This is tantamount also to senders calculating their own thumbnails for sending in advance of the main content - we are trusting the sender not to lie about the content in the thumbnail.  Whereas currently thumbnails are calculated by the receiving homeserver to avoid this attack.
+
+However, this kind of phishing attack does exist whether we let senders pick their thumbnails or not, in that a malicious sender can send normal text messages around the attachment claiming it to be legitimate.  We could rely on (future) reputation/abuse management to punish users who phish (be it with bogus metadata or bogus descriptions).   Bogus metadata is particularly bad though, especially if it's avoidable.
+
+As a first cut, let's do #2 and have the receiver hit the API to calculate its own previews (as it does currently for image thumbnails).  We can then extend/optimise this to option 4 as a special extra if needed.
+
+API
+---
+
+GET /_matrix/media/r0/previewUrl?url=http://wherever.com
+200 OK
+{
+    "og:type"        : "article"
+    "og:url"         : "https://twitter.com/matrixdotorg/status/684074366691356672"
+    "og:title"       : "Matrix on Twitter"
+    "og:image"       : "https://pbs.twimg.com/profile_images/500400952029888512/yI0qtFi7_400x400.png"
+    "og:description" : "“Synapse 0.12 is out! Lots of polishing, performance & bugfixes: /sync API, /r0 prefix, fulltext search, 3PID invites https://t.co/5alhXLLEGP”"
+    "og:site_name"   : "Twitter"
+}
+
+* Downloads the URL
+  * If HTML, just stores it in RAM and parses it for OG meta tags
+    * Download any media OG meta tags to the media repo, and refer to them in the OG via mxc:// URIs.
+  * If a media filetype we know we can thumbnail: store it on disk, and hand it to the thumbnailer. Generate OG meta tags from the thumbnailer contents.
+  * Otherwise, don't bother downloading further.