feature request - maximum main time for leagues

[alexmitrani]

Hello. We're starting a "blitz" league for the fechgo community this month. I just realised there is a small issue: because the time specifications are minima, if we tried to use the regular #fechgo tag for the blitz games, all the regular league games would be valid for the blitz league! So, for now, we will use a different tag for the blitz league: #fechgobz.

This isn't a very satisfactory solution because:

1. if someone tags a regular league match with #fechgobz as well as or instead of #fechgo it will count towards the blitz league.
2. by definition the blitz league should have a maximum allowable main time limit, otherwise people can play non-blitz matches and they will count, when they shouldn't. Just a minimum time limit does not clearly define what type of matches should be valid. So we will define a maximum in the rules and ask people to self-enforce that part of the rules.

The feature request is this: please incorporate a maximum main time limit in the league options. That way we could set a sensible maximum such as 5 minutes or 60 seconds and revert to using the regular #fechgo tag for all fechgo leagues.

Please also see the related #376.

Thanks very much!

[climu]

One could add a max_main_time field on LeagueEvent model.

Then do not forget to update the related forms and make a check at check_validity.

UI wise a slider would be neat.

PR welcome.