set max-width to 100% and height to auto

[sdumetz]

max-width was already set in styles.scss#L83, but not in editor leading to inconsistent behaviour.

Moreover, max-width without height: auto leads to stretched images.

I created a new css file because the current content.ui.min.cssand content.min.css seems directly imported from tinyMCE and keeping them pristine will probably help future updates.

With those changes it is still possible to manually force images to stretch by specifying a different height in the image block popup.

[gjcope]

We've been working on a fix as well since there are several inconsistencies like this. But this seems reasonable. Thanks!