Add Configurable Default Storage for Hosted RepoMaker

When hosting RepoMaker on a webserver, it should be possible for the webmaster to easily configure a default storage option that each repository uses by default as main storage option.

This way, users can just create a repository and get it published automatically to the provided storage location. That should simplify the UX a great deal.

The default storage will initially be just a local folder in the filesystem, but could also be a remote host (or VM) that gets filled via rsync+ssh.