Add event leaderboards

[florczakraf]

requirements from @lisek573:

• [ ] pick some songs that are going to be a part of the event
• [ ] select start and end date
• [ ] it has to be opt-in for players (I guess that people can join retroactively while the event is still live)
• [ ] I think we could also have an option to explicitly require no C-MOD for some songs just like ITL did (this requires #24 to be completed first)

Open questions:

• [ ] how do we want to initiate an event?
• [ ] can there be more than one active events live at the same time?
• [ ] who can create an event?
• [ ] how to add a song to the event? (some searching feature would be cool)

[florczakraf]

as already mentioned in #46, this could also benefit from a deferred score processing via a task queue

[florczakraf]

I've received an email with some suggestions related to this issue:

Hi! Have been enjoying playing on BS and it has a lot of great features.

I've been thinking of a great tool that would be cool to have in the ITG space which is an "events" function.

Some details:

Users could upload a set of ssc files to determine charts in an "event"
User plugs in parameters like date range of event, max participants (maybe), other relevant info. You could even go as far as to limit an event by gender, age, etc. Or maybe the event owner can manually approve entrants.
The pack is distributed in some way
When BS scores get submitted, if that score is eligible for an event and other users have opted into an event, it counts towards it.
Allows for multiple events, potentially many events going on at the same time. Perhaps limit the # that a user can create in a timeframe

I think this would add some differentiation between BS and future Groovestats developments since they seem focused on "mega events".