* Refactor `TouchHandler` to handle double-tap and swipe gestures.
* Renamed existing `onTouch` JavaScript methods to `onItemTouch` and
added `onContentTouch` methods for swipe gesture.
* Refactor double-tap. It's now a method in `TouchHandler` versus
anonymous functions in `listen()` method.
* Updated CSS classes.
* Added `touch-action` CSS for `.entry-content`.
* Renamed CSS classes for adding events in `TouchHandler`.
* Updated users settings to replace checkbox for double tap with select
for none, double tap, or swipe.
* Added database migrations for new gesture_nav option.
* Rename `users.double_tap` to `users.gesture_nav` and migrate
existing user settings.
* Updated translation files. (Non-English updated with Google
Translate.)
Resolves#1449, closes#1495
