Fix CSS: Flex on desktop breaks up ext Post galleries

[pdragun]

For desktop flex is breaking up extension Post galleries. Problem is that Swiper js lib takes all possible width and it breaks design up and doesn't work at all.

Swiper element has width like on img below and it resize the page to this size too, so page is really width.

Solution can be to change from flex to grid. Grid is used also for default flarum discussions & navigation and so Post galleries works there OK.