Add env to disable signups

[iHildy]

For self hosted users who have their url public and don't want ransoms finding it and leeching off of their server. They can just set an env of "DISABLE_SIGNUPS"=true and redeploy the app and it will confine to work with all already linked accounts/repos but no one can stumble across the url and link their GitHub account.

[google-labs-jules]

Jules is on it. When finished, you will see another comment and be able to review a PR.