Allow users of a specific GitHub organization to login

If `GITHUB_ORG_ID` is set in `ENV`, and someone attempts to login via GitHub without a prior account, one is automatically created for them.  This is both convenient and secure, by not necessitating an administrator to create accounts for every user before they can login.  It also restricts via the organization ID to prevent non-organization members from creating accounts.  After creating their account they can choose to edit their profile to an email different than their GitHub one.

Replacing #642 with this for a single squashed commit.