Admin can add/remove a todo item to a series

[php-coder]

As admin I want to add todo item to series for task that should be done. For instance, I may mark that series has image with bad quality to signalize that I need to change image in future.

Notes:

• I can add more than one todo for a series
• I want to have a special page with all my todos (where I also may mark todo item(s) as done)

[php-coder]

We can use comments for that purpose as a workaround.