Spun off from T194707#4620358
Some events only have a cTo make Category to go off of and no explicit participants list.filter more useful, We should support this use-case.we will add the ability to use Category filters without a Participants list.
When there are no participants but there are categories:
* Impose sane limits on the number of pages to process. This is the [[ https://github.com/wikimedia/grantmetrics/blob/master/src/AppBundle/Repository/EventRepository.php#L113-L115 | trickiest part ]].
* Count edits by all users, including IPs, but excluding bots.
* For the precomputed stats, maybe show "50 editors" and not "50 participants", as otherwise it would say 50 participants in one place, and then in the Participants section it will list 0.
* Show "na" or "–" for new editors / retention
* Add test to make sure the revision browser still works as expected (I always seem to forget about that page)