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 existing `gitlab:repo:check`
task which goes through all projects. 

See merge request !2080