Given that there is always a ton of `Entry` floating around, reordering its
field to take less space is a quick/simple way to reduce miniflux' memory
consumption.
I kept the `ID` field as the first member, as I think it's the most important
one, and moving it somewhere else would drown it in other fields.
Anyway, this still provides a reduction of 32 bytes per Entry:
```console
$ fieldalignment ./client/model.go 2>&1 | grep 203
~/v2/client/model.go:203:12: struct with 280 pointer bytes could be 240
$ fieldalignment ./client/model.go 2>&1 | grep 203
~/v2/client/model.go:203:12: struct with 248 pointer bytes could be 240
$
```
The same optimisation pass could be applied to other structs, but since they
aren't present in obviously great numbers during miniflux' life cycle, it would
likely require some profiling to see if it's worth doing it.
* 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
