Djblets has long had rate limiting support for user sessions, allowing

separate rate limits for anonymous users and authenticated users for

normal sessions and API. Outside of this, though, we had no mechanism

for rate limiting other operations.
This change introduces that generic rate limiting support via a new

djblets.protect.ratelimit module. This is a modernized port of the old

rate limit code, which can be used with arbitrary keys and validity

windows to provide rate limit functionality for anything.
The main function is check_rate_limit, which takes in a parsed or

string rate limit, a partial cache key, and a flag indicating whether to

increment the count toward the rate limit in cache (defaults to True).
The existing auth rate limiting code has been reworked as a wrapper

around this, focusing on the settings and checks for rate limiting

user sessions.

Unit tests pass.