R1:779744342865

Support invites in the registration and login flow

Summary:
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…