use settings_engine.themes_path for themes' url

Author: cpg

plugins/040-apps/app/views/apps/_is_installed.html.slim

@@ -25,7 +25,7 @@
   div.app-url= link_to(app.plugin.name, app.plugin.path, :target => '_blank')
 
 - if app.theme
-  div.app-url= link_to(t('manage_themes'), app.full_url)
+  div.app-url= link_to(t('manage_themes'), settings_engine.themes_path)
 
 - if app.webapp
   = render 'in_dashboard', :app => app