Migrate to a dedicated gitlab instance

I'm going to migrate Funkwhale's codebase and repositories to a dedicated GitLab instance very soon. We're currently using code.eliotberriot.com, which is my personnal instance, and as the project grows, it becomes less and realistic to keep it on my tiny Gitlab instance.

I've rented a dedicated server and spin up a VM to host the new Gitlab instance, which will be available at https://dev.funkwhale.audio. Code.eliotberriot.com will remain available

The migration workflow will be:

1. Stop code.eliotberriot.com
2. Backup code.eliotberriot.com
3. Restore the backup on dev.funkwhale.audio
4. Start dev.funkwhale.audio
5. Restart code.eliotberriot.com

At this point, we'll have a fork: two instances with the same database. People will be able to login on both instances. Then:

6. Setup a redirection from https://code.eliotberriot.com/funkwhale to https://dev.funkwhale.audio/funkwhale
7. Add a warning / description on code.eliotberriot.com to inform people about the migration
8. Close signups on code.eliotberriot.com
9. Add CI servers on dev.funkwhale.audio
10. Ensure emails are working
11. Ensure oauth on weblate is working from dev.funkwhale.audio
12. Ensure login from GitLab.com / Github is working properly
13. Ensure pages for Funkwhale project are served properly
14. Update all references to code.eliotberriot.com to dev.funkwhale.audio (documentation, contribute guides, readme, website, etc.)
15. Ensure backups are working

I've already tested steps 1 to 5 (which is why you can see a server up at https://dev.funkwhale.audio), so I'm pretty confident this will work :)