Detach repository group and path from the folder in disk

The simplest way to do this is by using an UUID for the folder name instead of using the current namespace/name.

There are multiple benefits of doing this, but I'll just mention 2 that I think are major:

1. Security: in order to identify a repo that you may want to get out of the disk you need to also compromise the database. This makes it harder to extract data from GitLab in case of a security breach.
2. Reusing group and project names after deletion stops being a problem - it's virtually impossible to get the same UUID.

This shouldn't be so much of a change to the application. But I think that the main issue will be to migrate all the projects around in the filesystem (a simple mv command would suffice) simply because it will take long.