fixes #2345 create piwigo standard pages

create new theme standard pages with tpl, css, js and images
update backend to handle pages
add config use_standard_pages
on update set config to false, on install set config to true
add standard pages to no be ingored in git

admin/themes_installed.php

@@ -59,7 +59,7 @@ $tpl_themes = array();
 
 foreach ($themes->fs_themes as $theme_id => $fs_theme)
 {
-  if ($theme_id == 'default')
+  if ($theme_id == 'default' or $theme_id == 'standard_pages')
   {
     continue;
   }