preserve accents in fields containing Twig expr. using unicode

[drzraf]

When a field contain accentuated characters, reduce the risk of messing with it by passing unicode characters unescaped.
Twig will deal with them. And fewer backslash-escaping problems will arise.

In order to allow blueprints fields to contain Twig expression, processString() is used.
Ideally it would deal with object/array by processing (= Twig-rendering) inner elements recursively.
But what it instead does, in order to flatten the value, is json_decode(Twig::render(json_encode($value)))

If someone use such a field value: {{ url.uri ~ 'é' }}, it will be transformed to {{ url.uri ~ '\u00e9' }}.
And Grav\Common\Twig::processString calling $this->twig->render() with that string returns localhostu00e9 whose accent won't be restored after json_decode().

I don't know why Twig calls seems broken. But using at least JSON_UNESCAPED_UNICODE is a safe enough solution that quickly reduces problem's surface.