RepositoryForkWorker uses absolute repository paths

RepositoryForkWorker has several problems:

it takes absolute repository paths from sidekiq. This needs to become a 'storage name + relative path' combination or perhaps just a project ID. Be careful: this has to be done in a backwards compatible way
it calls GitlabShell#fork_repository with absolute disk paths, those need to become storage names

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information