Banner cover images

Adding capacity to update hero images to static pages, outputting to page with fallback, and styling for both mobile/desktop.

@angaither 

CC: @barryclark @DFurnes 
@angaither not sure what the `$page->key = $filename;` was for but I updated it to store the path to the image in the db column item. Not that this ever really gets hit since we seed it all, but :goat: