Swipe gesture on mobile

[nolanlawson]

Swipe left and right on a timeline to switch columns, so you don't need to tap the button in the navigation header.

I avoided this because I found that the react-swipeable-views on Mastodon sometimes has false positives on certain browsers. E.g. on mobile Firefox you scroll down, but it's interpreted as a swipe, which is confusing. Also touchmove events can block scrolling.

I think it may be possible to do this performantly and accurately with Pointer Events, but this needs some investigation first.