[WIP] UX improvs

No issue

This is a set of small styling fixes that I think make mobile usage better, and a couple desktop things.
I've purposefully left them as separate commits so I can remove any of them if desired. Will sqaush when ready. :grin: 

They are as follows:

* Smaller padding on content previews for smaller width screens (not the editor)
* Adds `-webkit-overflow-scrolling: touch;` everywhere we have `overflow: hidden;` for native feeling scrolling on (many) mobile devices
* Minor adjustments to `<pre>` elements so lines of code don't wrap – scroll to the side to view it all
* Prevent the whole page bouncing up and down when you get to the top of bottom of a scrollable element