This adds any OAuth login buttons to the invite signup page, stores the invite code for the flow duration, and associates the new user with it once successfully registered.
It enables invite-only instances with OAuth-based registration.
This makes it possible to complete OAuth signup without creating a
password on the WriteFreely instance.
A user can then add a password to their account through their Account
Settings page without any admin action (all of this logic is already in
place).
Ref T715 T712
This uses the given username as the Display Name / Collection Title if a
user doesn't give one -- as might happen when authenticating with
Write.as.
Ref T712
Having a `Username` field populated in the page data tells the base
template to display navigation that only a logged in user should see. So
this renames the field to `LoginUsername`, similar to our login.tmpl
page.
Ref T712
