AO3-6277 Make email and the Matching page reflect it when match generation results in no matches

[slavalamp]

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Issue

https://otwarchive.atlassian.net/browse/AO3-6277

Purpose

When a gift exchange's match generation results in no matches, sends a "No potential matches found" email and makes the Matching page reflect the matching status appropriately too.

Also prepares some other Matching page states for localization, I got a bit carried away...

Credit

slavalamp (they/them)

[slavalamp]

I'm the procrastinator and forgot what else I wanted to do by now, so ready for review I guess. It might needs more tests?

I couldn't actually get the "No potential matches were found" page to show up locally consistently (sometimes it becomes "No potential matches yet!"), but the test succeeds every time I run it, so not sure if that's an issue with the code... though i also had web and tests actually behave differently earlier...

Also it resets the "no matches found" status and allows re-generating matches every time when any challenge settings get updated, even if they wouldn't have an effect on the ability to generate matches, bad idea or doesn't matter?