Remove lazy loading of images in /admin/selection

Some images don't show in the page if lazy loading is activated.
Put the list of categories at the bottom

In the /admin/selection the list of categories was moved to the bottom, because in some cases it was floating in a bad place at the right side of the page. This cannot be solved.
Show more columns and separate better the categories

Show a box around the categories to separate better from the pins.
Show 4 columns of pins instead of 3.
Separates the categories

Put a separator between each category in the /admin/selection.
Check the parent category when checking the child category
Merge pull request #92 from camilotorresf/pin-loaders

Fixes to /admin/selection/