Potential matching speed fix (issue 2869)

RELEASE NOTES: We have now significantly sped up the generation of potential matches for challenges, and added an option to regenerate potential matches just for individual signups (so if you are a challenge mod and you end up with only a few people who didn't match to anyone, you don't have to regenerate matches for everybody). The potential matching page should also now be easier to use, and additional help files have been added to explain the process better. 

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https://code.google.com/p/otwarchive/issues/detail?id=2869

Please see the gcode issue for detailed info! The short version is that this drastically speeds up the generation of potential matches (3 days for a big challenge ---> 14 hours) and allows for regenerating potential matches for individual signups (so all potential matches don't have to be regenerated just because one person didn't have any potential matches and their signup had to be tweaked).

I've also done a bunch of cleanup and refactoring on the potential matching page.