Add functionality to create teams

[punchagan]

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[punchagan]

Sorry for the delay in reviewing this PR. I've cleaned up this PR to add a single commit that adds the initial functionality.

There's still some work left, before I'd be comfortable merging this.

• I think we should trigger the forming of teams using another API call - users can click a button to create teams. And ideally this would be done after there are a significant number of players willing to participate. Any RSVPs that happen after the formation of teams can be automatically added to a team.
• I'm not sure how to handle cancellation of RSVPs - if we modify the teams, I think we would like to send a notification to users or something, but this could also be a feature that we add later. We should at least indicate in the UI when an RSVP has been cancelled, even if we don't change the teams. Let humans deal with that, for now.
• I think we don't want to always form pick-up teams, and may be add a flag on the event to indicate if we should allow forming pickup teams. But, this I guess is optional if we make the forming of pick-up teams be triggered by another API call.