Giving a Team access to a repo does... nothing useful (BB-7113)

I'm trying to give a dev team outside my organisation read access to a few repos. As I'm not a member of their team, I can't give them Group access, but the "User" picker allows me to type in a Team name even though it doesn't appear in the dropdown.

Of course, adding a Team doesn't given the team members access to anything.

We should fix two things:

1. Make it clear in the User picker that adding a Team won't work (or make it work)
2. When we decide that we won't make it work, add a method to get permission from a Team to give one of its Groups permission. In my case I don't even need permission to browse the external Team's Groups - I just need the ability to add the group to my repo.