FIX: preview theme not working consistently

Avoid flash, this makes debugging much simpler as well.

Additionally URL now clearly shows you are previewing a theme.

app/controllers/admin/themes_controller.rb

@@ -5,10 +5,14 @@ class Admin::ThemesController < Admin::AdminController
 
   skip_before_action :check_xhr, only: [:show, :preview]
 
+  def self.whitelist_theme_key(user)
+    "whitelist_theme_key_#{user.id}"
+  end
+
   def preview
     @theme = Theme.find(params[:id])
-
-    redirect_to path("/"), flash: { preview_theme_key: @theme.key }
+    $redis.setex(Admin::ThemesController.whitelist_theme_key(current_user), 60, @theme.key)
+    redirect_to path("/?preview_theme_key=#{@theme.key}")
   end
 
   def upload_asset