Move omniauth form partial to a component

This way we simplify the view a little bit and replace some slow system
tests with faster component tests.

app/components/devise/omniauth_form_component.rb

@@ -0,0 +1,22 @@
+class Devise::OmniauthFormComponent < ApplicationComponent
+  attr_reader :action
+
+  def initialize(action)
+    @action = action
+  end
+
+  def render?
+    oauth_logins.any?
+  end
+
+  private
+
+    def oauth_logins
+      [
+        (:twitter if feature?(:twitter_login)),
+        (:facebook if feature?(:facebook_login)),
+        (:google_oauth2 if feature?(:google_login)),
+        (:wordpress_oauth2 if feature?(:wordpress_login))
+      ].compact
+    end
+end