Add better support for mobile devices

[joelspadin]

Some current issues with the site on mobile:

• The HTML draggable property doesn't work when using my browser's mobile emulator, so it is not possible to add objects to the scene.
• If the arena area is scrollable, dragging on the background around the arena works for panning, but dragging on the arena itself does not.
• The layout completely breaks on very small screens, even in landscape mode.
  
• It is probably not possible to make the app very usable for editing a plan on anything smaller than a tablet, but with an option to hide the sidebars, it could be reasonable to view a plan on a small screen.