Thisi and the following commit follow the approach used in
3e2ad84bbe.
First migration requires a server restart - after that any new realms
will be created with the columns set.
The following migrations are in the next commit:
Second migration does a backfill for older realms and can run in the
background while the server is operating normally.
Third migration enforces null=False now that all realms have the columns
set.
