Limit the rate of logins.

This pull request adds rate limiting to our authentication. This covers:
* logging in from LMS
* logging in from Studio
* logging into admin sites
* tests for some of our login flows
* removing an outdated settings file

These things are deliberately not tested after talking to Test Engineering:
* tests for functionality that we do not use (admin on LMS)
* SSL login / OpenID

This does not cover additional work needed for logging, since that relies on pull request 539 to implemented.