Images on mobile can be wider than the screen

[TrueBrain]

Initially images on mobile were set to width: 100% to counter this, but as of #141 this is no longer the case.

It turns out there are images that are uploaded with a higher resolution than shown in the wiki. Setting width to 100% causes them to be shown at their original resolution, breaking layouts in places.

We need to find a proper solution to fix both. Possibly the latter problem can be fixed easily, so the first can be added again, but I haven't really looked into this.