allow users to create explicit Teams

[vanderhoop]

As a user who has completed a retro with other participants but where those participants don't make up a particular team,,
I want to be able to create a persistent team from the retro's participants,
So that we can review our particular action items and progress over time.

By Way Of Explanation: we have purposefully not made users create a team when joining RemoteRetro, as this would act as an annoying barrier for entry for new users who just want to see what the RR retro experience is like, and we place a premium on keeping the barrier for entry as low as possible. However, there are a number of features that not having teams keeps us from accomplishing, such as previous-retro action item review (as there is no deterministic way of knowing which retro was the last for a team, as the members might belong to multiple teams) and taking in-sprint notes that get imported into a new retro.

Acceptance Criteria:

• when the facilitator of a retro not associated with a team closes a retro, they are invited to "Create a Team!" from the retro's participants.
  • a team has an id, a name, and users, and retros, and an admin
    • users can belong to many teams
    • retros belong to a single team

Open Questions:

• should the invite to create a team go out via email?
  • users are pressed for time when a retro closes, so asking them to do more might be silly
• alternatively, the ask can take place right in-app?
  • the only user input needed is the team name, which could be supplied very quickly. everything else can be handled by the app.
• for implicit teams that have had multiple retros before joining an explicit team, how do they associate those old retros with a team? we can't do a blanket association, as a user might have taken part in a number of retros with different teams

Helpful Links:

• Documentation for Ecto.Schema

[vanderhoop]

This will unblock #347