[FEATURE] Make relations more performant and flexible

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.
[TASK] Create update script

Adds the newly introduced fields files and categories.

Also relations from files to categories has to be moved to mn relation table to dont break them.
Merge pull request #1 from benjaminrau/refactoring

Refactoring of dd_download - more flexible and performat relations