Full Admin Panel

I wrote @KrauseFx an email the other day about this. After playing with this, the idea is great, however, I (and I am sure many other would feel the same) don't like the current flow where anyone can actually add themselves to TestFlight. Before Apple bought them, you were able to share a link to a page where people could sign up and join, but you had to approve people.
1. Enable or Disable Signups
2. Set a date signups are open
3. Set a limit on signups
4. Enable automatic TestFlight enrollment
5. Approve/Deny Signups.

The first three are pretty self explanatory, but let me go into detail about the 5th. Basically, a system I would use would work like this.
You create the heroku app, setup everything like you do now, but you also create an admin account(s) which would allow you to manage the app persay. If Option #4 is disabled and you have it set to approve or deny signups, you would be presented with a list of people who signed up via the form. From here you can then approve users, thus enrolling them in TestFlight.

It's just an idea so if anyone has any suggestions or comments, feel free to leave them.

No files, tests, or entry points are named. Start by mapping the existing signup and TestFlight-enrollment flow, then define the admin and signup requirements before implementation; done would require an agreed design covering signup controls, limits, approval, and enrollment.

Written by the indexing model from the issue text.