Upgrading Mastodon to our fork

Due to some unforeseen issues, we aren't going through with this. Mastodon has been restored to upstream v4.1.2.

Posted 7 Apr at 09:37pm CEST.

The Mastodon server will be temporarily unavailable during this time to move to our own fork of Mastodon.

This will:
- Add mastodon-bird-ui as a selectable theme.
- Add support for thread lines on mastodon-bird-ui
- Increase max toot length to 10,000
- Make it easier for us to make changes in the future

Posted 7 Apr at 08:55pm CEST.