Custom subscriptions does not work with postgres

Issue Summary

You get an error that the 'not null' constraint of 'module' is violated.

The problem can be fixed by uninstalling notifications_extra, applying this patch, which adds a default value of '' to 'module', and reinstalling notifications_extra.

I did not test with mysql, but presumed it worked. I did not include a db update action, as I presumed that postgres users would not have the module installed, since it does not work, and mysql users would not need the database change, since the code presumably already works.