Turn off "Other" notifications by default in Juno

We want to encourage apps to use the notifications API properly with the freedesktop ID and everything. In Loki, we grouped all non-compliant apps together under "Other" in the Notifications plug. In Juno, I think we should turn those notifications off by default.

This means that out of the box:
1. Apps using the API properly work as expected.
2. Apps not using the API properly can update to use the API properly to get notifications back on by default.
3. Apps not using the API that don't update can still be opted into by the user by switching on "Other"