Allow editing of match dates (and other data)

[jslater89]

This is actually quite a lot harder than it sounds, because it means reencoding matches in my own storage format, or learning to output a (subset of) USPSA's.

Might make the match cache smaller, though.

[jslater89]

This is possible, with the new database. I think I should set a 'canonical' flag on matches pulled from a source, and require users to edit a copy rather than allowing edits of the base match.

Probably also good to track last edit time, so we can prompt for full recalculation of rating sets that include edited matches.