RateLimiter improvements: runtime configurability, no default limits in tests

Runtime configurability of RateLimiter. Refactoring. No default rate limits in tests.

Compile-time settings (previous approach) were not only inconvenient (required restart to change limits) but also caused complications in tests (e.g. failures temporarily fixed in https://git.pleroma.social/mfc/pleroma/merge_requests/70/diffs?commit_id=c61f88adabb740e3e5b18b50e7596f20accfa2db). This MR defines no rate limits in test environment (configurable for rate limiter tests since settings are no longer compile-time), so the above issue won't rearise.