Ref T7152. This substantially completes the upstream login flow. Basically, we just cookie you and push you through normal registration, with slight changes:
- All providers allow registration if you have an invite.
- Most providers get minor text changes to say "Register" instead of "Login" or "Login or Register".
- The Username/Password provider changes to just a "choose a username" form.
- We show the user that they're accepting an invite, and who invited them.
Then on actual registration:
- Accepting an invite auto-verifies the address.
- Accepting an invite auto-approves the account.
- Your email is set to the invite email and locked.
- Invites get to reassign nonprimary, unverified addresses from other accounts.
But 98% of the code is the same.
