Big images in editor can overflow out of editor's container

[EvitanRelta]

When the markdown is hidden, and u load in big images, they fit within the editor's container as expected.
However, when u then show the markdown (and thus potentially halfing the editor's width), the image does not resize to fit within the container (potentially overflowing into the markdown on the right).

Now with #129 implemented, reloading while the markdown is shown will keep it open. And the image will resize to fit the halfed container, but upon closing the markdown, the image doesn't resize to fit the now bigger container: