Split form.css into form-base.css and form-admin.css, drop dead upload-progress code

Also introduces $extraCssAdmin support in head.php for admin-only
stylesheets (form-admin.css, filepond CSS, system.css). Admin pages
now use $extraCssAdmin for admin-only assets and $extraCss for
shared stylesheets like form-base.css.

app/public/admin/parametres.php

@@ -74,7 +74,7 @@ if (empty($_SESSION['csrf_token'])) {
 }
 
 $isAdmin = true; $bodyClass = 'admin-body';
-$extraCss = ['/assets/css/system.css'];
+$extraCssAdmin = ['/assets/css/system.css'];
 require_once APP_ROOT . '/templates/head.php';
 include APP_ROOT . '/templates/header.php';
 include APP_ROOT . '/templates/admin/parametres.php';