Limit the rate of logins

This pull request adds rate limiting to our authentication backend. 

This pull request covers:
* rate limiting log in attempts from LMS
* rate limiting log in attempts from Studio
* rate limiting log in attempts into admin sites
* tests
* hiding warnings when you don't want to see them
* removing an outdated settings file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 sites on LMS)
* SSL login / OpenID

This does not cover additional work needed for logging, since that relies on pull request 539 to implemented.

@brianhw @nedbat , review?