Issue macaroons as opaque auth tokens

This just replaces random bytes with macaroons. The macaroons are not
inspected by the client or server.

In particular, they claim to have an expiry time, but nothing verifies
that they have not expired.

Follow-up commits will actually enforce the expiration, and allow for
token refresh.

See https://bit.ly/matrix-auth for more information

tests/utils.py

@@ -44,6 +44,8 @@ def setup_test_homeserver(name="test", datastore=None, config=None, **kargs):
         config.signing_key = [MockKey()]
         config.event_cache_size = 1
         config.disable_registration = False
+        config.macaroon_secret_key = "not even a little secret"
+        config.server_name = "server.under.test"
 
     if "clock" not in kargs:
         kargs["clock"] = MockClock()