feat: let owners leave teams if nobody else is there

[jimniels]

If you create a team (cause hey, people click around, try things out) you're forever stuck with it! I have some teams in prod I can't get rid of

We should allow people to just leave teams, even if they're the owner.

If there are other people in the team, that presents a bit of a problem because you need to make sure someone else has ownership. We could punt on that one for now.

But if you are part of a team that has no other members, you should be able to just leave it (and it becomes, essentially, and orphaned team in the database).