Update documentaton and scripts for a single codebase

In the single codebase setup we no longer have a gitlab-ee and gitlab-ce repository. Instead, all development happens in the gitlab-org/gitlab repository, with gitlab-org/gitlab-foss just being a mirror.

We need to update all scripts and documentation as follows:

1. Any links pointing to the gitlab-ce and gitlab-ee repositories should be updated to point to gitlab-org/gitlab instead.
2. We no longer need separate guides for setting up CE and EE separately, as they are now one. We should mention how one can set up a copy of gitlab-foss if they want to (this is basically the same as setting up a separate EE installation)
3. We probably need to rewrite some parts of the documentation for this new setup