Refactoring of dd_download - more flexible and performat relations

Category property on File model becomes a m:n relation to category.

Introduced files property on Category enables us work with the attached files also on category.

Now inline editing of files on category record is possible.

Template has been changed because we can now iterate over files property on Category to show all files and there is no longer need to to several iterations within other iterations.

Also provided an updateScript to migrate relations from files to categories to mn relation table.