Support basic swipe gestures in Pholio

Summary:
Ref 2700. No sexy animation yet, but allows you to swipe left and right to switch photos. Generally feels pretty good to me.

Also fixes a left/right but and a bug where taps could be interpreted as gestures and simplifies some touch code.

derpdog

Test Plan:
Swiped left and right in Pholio.

{F35239}

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5309