Add "Join Team" button which triggers moderation workflow

[batpad]

High level: We want a better workflow for users to be able to discover teams and request to join them, without being explicitly invited.

There's likely a few different options here, one of which is:

• Add a "Join Team" button on Teams (perhaps if the team moderator choses for their team to be "joinable"?)
• User clicks Join Team and and a request is sent to moderator.
• If the moderator approves, user is part of that team.

There's a few complexities here, including the fact that we have no good way to notify users via email or so when they have been accepted to a team. It also does introduce more complexity for "types" of teams if we want some teams to be Fully Public to Join, some Joinable with Approval, some with no public Join button at all, etc.

This likely needs a bit of thought, but adding the ticket here based on discussion - @jenningsanderson @LanesGood @kamicut will be great to add thoughts and we can see how this fits into our work-plan.

[LanesGood]

Also see: #11 #81