Cleanup: Move asset mixins to includes/mixins

We use a separate file for mixins to make views more readable. LP#15202477
Add full screen categories menu

Append menu on the right side of the screen. It overtakes all screen focus qua an overlay on the toggle element (may be moved to another).

Also appends natmus.js to allow for jQuery.

LP#15202477
Initial setup of Category menu

Get ready to show submenus in category menu. Probably requires some kind of Angular $watch in app.js.

Also style adjustments for toggle icon.

LP#15202477