Treat API requests from the frontend as web traffic in the rate limiter

This will allow us to impose stricter rate limits for general API
traffic, without affecting interactive API requests made by the
frontend during normal GitLab usage.

The frontend requests are identified by the inclusion of a CSRF token
in the headers.

Other rate limits that only affect a subset of API requests (e.g. the
Files and Packages APIs, or protected paths) still take precedence,
i.e. requests for these paths will always be matched even if they
include a CSRF token.

Changelog: changed