Comment gesture to go back

**The Problem**
When browsing one-handed with my right, navigating from comments view back to posts requires me to move my short, stubby thumb all the way to the left edge of the screen to swipe. That requires some acrobatics with modern device sizes.

This can (eventually) be solved by #101, but that is (1) currently waiting on ionic, and (2) requires losing _both_ Left Short & Long gestures.

**Proposed solution**
Allow comment gestures to be configured with a "Navigate Back" action. This allows people to configure, say, Left Short to go back, without losing the Left Long gesture.