mod_authentication: Refactor twitter/facebook logon and signup code.

Now uses a popupwindow for the authentication.
Twitter and Facebook settings are combined in the admin underneat the 'Auth' menu.
Authenticated users are immediately signed up, no extra checks.

(cherry picked from commit 1c1f7b3c3b9905741a171f9a5439b181e0a918a1)

Conflicts:
	modules/mod_facebook/templates/admin_facebook.tpl
mod_facebook/twitter: change bootstrap3 classes in bootstrap2