FEATURE: Per-plugin settings buttons, "Enabled" column

Also, added enabled_site_setting to the Poll plugin so it shows up properly.

app/serializers/admin_plugin_serializer.rb

@@ -3,7 +3,9 @@ class AdminPluginSerializer < ApplicationSerializer
              :name,
              :version,
              :url,
-             :admin_route
+             :admin_route,
+             :enabled,
+             :enabled_setting
 
   def id
     object.metadata.name
@@ -21,6 +23,18 @@ class AdminPluginSerializer < ApplicationSerializer
     object.metadata.url
   end
 
+  def enabled
+    object.enabled?
+  end
+
+  def enabled_setting
+    object.enabled_site_setting
+  end
+
+  def include_enabled_setting?
+    enabled_setting.present?
+  end
+
   def include_url?
     url.present?
   end