Tie characters to a registered fork #115

Character.fork_compatibility is a free text string right now, so anything can claim to be any fork. It needs to point at a registered fork from #114.

Existing characters get mapped to the "Unitystation" fork by default. An account can still have characters on lots of forks.

Servers can read characters from any fork but can only write to their own.

Who ever does this job, ask bod whether fork_compatibility becomes a foreign key, a list of supported forks or stays as a text key?