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

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

lib/plugin/instance.rb

@@ -299,8 +299,12 @@ class Plugin::Instance
     end
   end
 
-  def enabled_site_setting(setting)
-    @enabled_site_setting = setting
+  def enabled_site_setting(setting=nil)
+    if setting
+      @enabled_site_setting = setting
+    else
+      @enabled_site_setting
+    end
   end
 
   protected