Signed-off-by: prichier <pascoualito@gmail.com>

Signed-off-by: prichier <pascoualito@gmail.com>

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 passes in the correct field named GitlabDisplayName.

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.

- Move them above local login form
- Restyle as side-by-side buttons

Ref T712

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.

- New editor nav
- New backend nav
- Support for drafts
- Different footers on backend