Submission revisions

Issues

• Only units that are translated or fuzzy get a creation submission
• ATM submissions are created separately to when units are saved which means a lot of duplicate code
• Whether unit.save triggers a submission creation deps on _implicit_updated vars
• Risk of code updating units without creating submissions
• No submission recording ATM for changes to context/location - we need to record all diffable unit changes

Initial steps

• Add a signal/handler for triggering submission recording when indiv. unit has changed
• Remove the submission recording from other places
• Add a signal suppressor to allow store submission recording
• Ensure that all unit create/delete events are recorded