Add external authenticator (social login) API

Allows registrations to be completed with a pre-confirmed email address
and no password.

less/forum/app.less

@@ -10,6 +10,7 @@
 @import "EditUserModal.less";
 @import "Hero.less";
 @import "IndexPage.less";
+@import "LogInButton.less";
 @import "LogInModal.less";
 @import "NotificationGrid.less";
 @import "NotificationList.less";