Ensure local actors preferredName don't already exist

Ensure local actors preferredName don't already exist

Before applying this commit, check you don't have duplicates local
actors in your database:

select "preferredUsername" from actor where "serverId" is null group by "preferredUsername" having count(*) > 0

If you have some results, it seems you have duplicate channels/accounts.
For every entry, you'll have to change the preferredUsername of the
entry you want (so they are unique). The updated actors could have some
federations issues. Sorry.