Swipe

The behavior of the gallery on mobile is pretty bad. It first requires a tap on the previous/next button, which we must "guess" exist, and the a tap on the button.

Consider getting rid of the navigation buttons altogether and detecting swipes instead.

Here's an example using my [tiny lib](https://github.com/marcandre/detect_swipe). Consider bundling it?

Builds on all other PRs up to #22 (sorry, and sorry for the avalanche of PRs).