Move the "email unsubscribe" resource, refactor the macaroon generator & simplify the access token verification logic. (#12986)

This simplifies the access token verification logic by removing the `rights`
parameter which was only ever used for the unsubscribe link in email
notifications. The latter has been moved under the `/_synapse` namespace,
since it is not a standard API.

This also makes the email verification link more secure, by embedding the
app_id and pushkey in the macaroon and verifying it. This prevents the user
from tampering the query parameters of that unsubscribe link.

Macaroon generation is refactored:

- Centralised all macaroon generation and verification logic to the
  `MacaroonGenerator`
- Moved to `synapse.utils`
- Changed the constructor to require only a `Clock`, hostname, and a secret key
  (instead of a full `Homeserver`).
- Added tests for all methods.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/12986.misc b/changelog.d/12986.misc
new file mode 100644
index 0000000000..937b888023
--- /dev/null
+++ b/changelog.d/12986.misc
@@ -0,0 +1 @@
+Refactor macaroon tokens generation and move the unsubscribe link in notification emails to `/_synapse/client/unsubscribe`.