Can't request scrimmages in a team with a name of a previously-deleted team

[jmerle]

Here's what I did:

1. Create a team named '∞'
2. Leave my team
3. Create another team named '∞'
4. Request a scrimmage from someone

This is the error I get (it's an HTTP 400 response):

The "it returned 2" becomes "it returned 3" if I re-create my team with the same name, and 4 if I do it again.

Here's how I think it can be fixed (I have no Django experience so I got no idea how to code this):
When a team is deleted, team.deleted is set to True. When the ScrimmageSerializer is used to check whether the team names of the teams participating in the requested scrimmage are valid, it requests all Team objects and tries to match by name. Since there are now multiple teams with name "∞", this returns multiple results which is not allowed. I'm guessing this can be fixed by making sure the Team.objects.all() results are filtered to only include teams with deleted set to False.

[joshuagruenstein]

@jsegaran @kelvin-lu @arvid220u can one of you look into this?