Implement Org Team Memberships API

Implements the new [Team Membership API][blog].

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@joeyw @jakeboxer

[blog]: http://git.io/a9jglQ
This doesn't necessarily invite a user to a team. The way it works is:

If the user is already an org member (meaning they're a member of at least one other team on the same org), they'll be added to the team.

If the user is not an org member (which means they aren't a member of any of the org's other teams), they'll be sent an invitation.