Rate limit Users API endpoints behind FF

There are a couple of endpoints in the Users API that need to have a rate limit added. The endpoints and suggested limits are listed below:

Based on this comment #421905 (comment 1832435121) we'll be rate limiting based on user if authenticated and on IP if unauthenticated. We can have the same values for both limits.

Endpoint	Requests per minute Limit
GET /users/:id/followers	100
GET /users/:id/following	100
GET /users/:user_id/status	240
GET /users/:user_id/keys	120
GET /users/:id/keys/:key_id	120
GET /users/:id/gpg_keys	120
GET /users/:id/gpg_keys/:key_id	120

We should make sure that this feature is behind a feature flag, so the groupauthentication can roll it out and watch for any unintended consequences.

This Kibana dashboard can help to see the current usage pattern.

See also #421905 (comment 1832435121), #421905 (comment 1806884841) and #421905 (comment 1820899320).