Improved UCP one-step settings update

Before: it would rely on getAccessToken() method + $mybb->input['code']
presence. It could be easily spoofed with a query string, provided a
valid access token.

Now: when you update your settings, if you aren't logged in w/ Facebook
you are silently logged in and a fb_islogginin value is stored within
the $_SESSION array. Afterwards, when you are redirected to the
settings page again, $mybb->request_method isn't POST anymore but the
settings update process begins regularly thanks to the fb_islogginin
value detected. Settings are updated, your data is being synced, and
fb_islogginin value is removed from your session.

Result: better way to handle settings updates.