Multiple demo import layout

Maybe this would really be a better layout for multiple imports?

[x] build the grid import layout
[x] where should we present the notice text?
[x] Once the import is run, hide all other imports + navigation to display the "progress"?
[x] Add a popup confirmation when clicked on import
[x] Add filter to disable the confirmation popup, so that theme users can disable it
[x] Enable items to belong to multiple categories
[x] Fix navigation responsiveness and multiple lines of categories/nav items
[x] Check styling and functionality again!
[x] Make the confirmation popup responsive?
[x] FadeIn/Out on category navigation changes
[x] Add Marko to the contributors
[x] Code review/refactoring + file changes needed (remove the old multi predefined import layout)

awesomemotive / one-click-demo-import