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 moves logic for determining whether or not to show a "Sign up" page
on /signup (i.e. because the `/` route shows another, non-signup page)
into the AppCfg.SignupPath() method. It also changes various signup
links to use this value.
Previously, a failed login would change the site-wide navigation so that
it looked like the user was logged in, even though they weren't. This
fixes that.
Keturah Dola-Borg
Matt Baer
Matt Baer
Kyle Robbertze
Nick Gerakines