Projects::ForkService delegates to this service almost entirely, but needed
one small change so it would propagate create errors correctly.
CreateService#execute needs significant refactoring; it is now right at the
complexity limit set by Rubocop. I avoided doing so in this commit to keep the
diff as small as possible.
Several tests depend on the insecure behaviour of ForkService, so fi them up at
the same time.
Previously this controller would in multiple places load tons (read:
around 65000) project and/or group IDs into memory. These changes in
combination with the previous commits significantly cut down loading
times of user profile pages and the Atom feeds of users.
