Developed rate-limit for failed login attempts through either the UI

or API. See review request 8768 to see it's implementation in Review

Board's Authentication form.

Create several unit tests for the login rate limiting feature that

test the rate limits as either custom or default values. They are

found in djblets/auth/tests/test_ratelimit.py and in

webapi/tests/test_api_auth_backend.py.