This makes this value much easier for a server admin to change than it
was when embedded directly in the code. (Note this entire mechanism
already only applies on a server open for anyone to create a realm.)
Doing this also means getting the default out of the database.
Instead, we make the column nullable, and when it's NULL in the
database, treat that as whatever the current default is. This better
matches anyway the likely model where there are a few realms with
specially-set values, and everything else should be treated uniformly.
The migration contains a `RenameField` step, which sounds scary
operationally -- but it really does mean just the *field*, in
the model within the Python code. The underlying column's name
doesn't change.
Greg Price