Make Alioth website repository read-only

This will need to wait until we have updated that site sufficiently to point users to GitLab.

To make the Alioth repository read-only, the following need to be taken care of:

• make update-htdocs pull from GitLab.com (by changing the clone's remote)
• make Alioth repositories read-only
• verify that the GitLab mirrors are up-to-date
• stop mirroring
• update the project description
• update the website (see website@02240289)

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.