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.

tests/unittest.py

@@ -315,7 +315,7 @@ class HomeserverTestCase(TestCase):
                         "is_guest": False,
                     }
 
-                async def get_user_by_req(request, allow_guest=False, rights="access"):
+                async def get_user_by_req(request, allow_guest=False):
                     assert self.helper.auth_user_id is not None
                     return create_requester(
                         UserID.from_string(self.helper.auth_user_id),