Move deduplication across from support

Anonymous feedback coming through the user satisfaction
and report-a-problem mechanisms is sometimes submitted twice. While
there are browser-side mechanisms to prevent double submissions, they can't
be prevented entirely. This change enables running cron tasks that remove duplicates.

The nightly deduplication functionality currently exists in the `support` app, but moved here as part of the app split and will be removed from `support` when this change is live. 

This PR also adds a worker for frequent deduping (every 5 minutes) so that today's feedback is deduped as it comes in. In theory, that makes the nightly deduping unnecessary but the nightly job is still there to sweep up any dupes in case the frequently run worker didn't run successfully at the specified time (eg due to MySQL issues).
I don't understand what this line is meant to be doing. Could you refactor it to make things clearer.