/series/{id}: admin should be able to add a comment

[php-coder]

[php-coder]

Instead of allowing admin to provide a comment during series import, let's make it generic and allow admin to add a comment to a series.

We also need to think about whether we allow update comment or not.

[php-coder]

We also need to think about whether we allow update comment or not.

We can start from showing a form only when there is no comment.