Add user repository integrity check rake task

Corrupt repositories and stuck lock files can cause weird issues in GitLab. Often we know which user is having these problems and then we have to go hunt down which repository is causing it. Several times recently that involved me running queries in the rails console to get an array of projects and then writing a quick Ruby script to loop through and run git fsck. This last time I also had to check for the existence of config.lock and ref lock files.

This rake task will eliminate all of those steps and allow an admin to simply specify a username.

I also added the lock file checks to the exist gitlab:repo:check task which goes through all projects.