UX improvements on Locales source plugin form

Been a minute, re-running tests to see if the existing ones are working yet.

This is a great improvement! I couldn't figure out why the locale source text search was broken, and then I found that it was a default submit issue ("choosing the TMGMT source" is the default when you hit enter because it's first), and that led me here. Marking major since the current experience is a trip-up.

+1 RTBC for patch #7. Hitting performs search on the entered text now.
Thank you.

These queries aren't going to fix themself on Postgres/sqlite, this needs to be updated to a merge request.