Implement "self-destruct" ActivityPub command

[Tobias Brunner]

To be able to release a domain for changing the ActivityPub software or to reinstall Takahe from scratch, a self-destruct command has to be sent so that it's properly removed from other ActivityPub servers.

See:

• superseriousbusiness/gotosocial#1697
• https://docs.joinmastodon.org/admin/tootctl/#self-destruct