Codeless projects / disable the repository

The ability to have a projects with for example just an issue tracker and/or a wiki. Requested by https://news.ycombinator.com/item?id=12145350 and a wish of Sid for years. This would make GitLab much more usable as an issue tracker or wiki for non-developers.

This you can enable/disable the repository under the project settings just like you do that for issues.

@dzaporozhets how much complexity would this add in the codebase?

/cc @JobV @markpundsack @jschatz1