Add Rake task to create/repair GitLab Shell hooks symlinks

What does this MR do?

Adds a Rake task wrapper around gitlab-shell bin/create_hooks script. This makes it easier for an administrator to repair hooks symlinks without worrying about where repository data is located, etc. Just run this task and it takes care of everything based on current configuration.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

bin/create_hooks was not well-documented. It requires parameters specifying where repository data is located. It also needs to be run by the git user. Wrapping it in a Rake task allows us to take current configuration in to account and makes it easier on the administrator.