System-wide awareness of viewport state

[CarlosNZ]

Part of Epic #1548

We'll need our React components to have awareness of the current viewport.

I'm thinking we take the useWindowDimensions hook I made for #1564 and turn it into a Global context (so we're not calling the same hook multiple times simultaneously). Then made the following additions to it:

• returns an "isMobile" boolean for other components to quickly know whether to display as mobile or not.
• add a little debouncing to the event listener so it doesn't fire off like crazy while window is being resized (as that'll cause a lot of re-renders)
• Anything else?

[CarlosNZ]

Closed by #1571