Remove restriction on guid length

This is more a request for comments than a pull request

I think it would be a good idea to switch GUID generation to UUIDs: https://github.com/MrZYX/diaspora/compare/uuid However our current database schema limits their length too much, so it would be a change breaking federation, the earliest point where we could do it is the next major release. What we could already do though is removing that restriction, reducing the impact in the migration phase.