Adds backend code for the mute users feature.
This is just infrastructure work (database
interactions, helpers, tests, events, API docs
etc) and does not involve any behavioral/semantic
aspects of muted users.
Adds POST and DELETE endpoints, to keep the
URL scheme mostly consistent in terms of `users/me`.
TODOs:
1. Add tests for exporting `zulip_muteduser` database table.
2. Add dedicated methods to python-zulip-api to be used
in place of the current `client.call_endpoint` implementation.
Abhijeet Prasad Bodas