Add swipe as option for gesture navigation between entries.

* 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

tests/user_test.go

@@ -88,6 +88,10 @@ func TestGetUsers(t *testing.T) {
 		t.Fatalf(`Invalid web app display mode, got "%v"`, users[0].DisplayMode)
 	}
 
+	if users[0].GestureNav != "tap" {
+		t.Fatalf(`Invalid gesture navigation, got "%v"`, users[0].GestureNav)
+	}
+
 	if users[0].DefaultReadingSpeed != 265 {
 		t.Fatalf(`Invalid default reading speed, got "%v"`, users[0].DefaultReadingSpeed)
 	}