Make GitLab backends repository the canonical location for code

To do so, the following need to be taken care of:

• make Alioth repositories read-only
• verify that the GitLab mirrors are up-to-date
• stop mirroring
• update the project description
• update Alioth references in the source (backends@f38a1731)
• update the website (see website@371e29ff)

Making the Alioth repositories read-only can be done by making the pre-receive hook on Alioth exit with a non-success status. The script should echo information on where push requests should be directed from here on and how to switch a local repository over.