Reduce Rack Attack false positives causing 403 errors during HTTP authentication
### What does this MR do?
This MR reduces false positives causing `403 Forbidden` messages after HTTP authentication.
A Git client may attempt to access a repository without a password. If it receives a 401 error, the client often will try again, this time supplying a password. The problem is that `grack_auth.rb` considers a blank password an authentication failure and increases a Redis counter each time this happens. With enough requests, an IP can be banned temporarily even though previous attempts may have been successful. This leads users to see `403 Forbidden` errors until the ban times out (default: 1 hour).
To reduce the chance of a false positive, this MR resets the counter upon a successful authentication from an IP.
In addition, this MR logs when a user has been banned and introduces the ability to disable Rack Attack via a config variable.
### Are there points in the code the reviewer needs to double check?
rack-attack v4.2.0 doesn't support the ability to clear counters out of the box, so `rack_attack_helpers.rb` includes a number of monkey patches to make it work. It looks like this functionality may be added in v4.3.0. I've also sent pull requests to rack-attack to add the functionality necessary to delete a key.
Each time an authentication is successful, the Redis counter for that IP is cleared. I deemed it better to clear the counter than to allow for blank passwords, since the latter seems like a security risk.
### Why was this MR needed?
It was quite difficult to figure out why users were seeing `403 Forbidden`, which is why the log message was added. Users were getting a lot of false positives when accessing repositories with HTTPS. Including the username in the HTTPS URL (e.g. `https://username@mydomain.com/account/repo.git`) caused authentication failures because while the git client provided the username, it left the password blank, leading to an authentication failure.
### What are the relevant issue numbers / [Feature requests](http://feedback.gitlab.com/)?
See Issue #1171
https://github.com/kickstarter/rack-attack/issues/113
See merge request !392
