Rate Limiting GitLab API Calls

We should implement rate limiting in the front-end proxy to limit the number of calls per second that we process from a given requesting client. Exceeding that rate limit would return a 429 "too many requests" and encounter a brief back-off period for the client. The number that @pcarranza has posited is 3 requests per second.

/cc: @pcarranza @stanhu @jacobvosmaer-gitlab @jnijhof @ayufan