Add external authenticator (social login) API

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

less/forum/LogInButton.less

@@ -0,0 +1,11 @@
+.LogInButton {
+  &:extend(.Button--block);
+}
+.LogInButtons {
+  width: 200px;
+  margin: 0 auto 20px;
+
+  .LogInButton {
+    margin-bottom: 5px;
+  }
+}

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";