Let users manually set the width and height of an image either using a
text field or by a preset percentage amount.

The resizing tools we use size a parent div on the image. The height:
auto responsive CSS we inherit from up higher messes with this. So this
push adds an explicit 100% height on resized images.

Co-authored-by: Joen Asmussen <joen@automattic.com>

Setting an explicit line height on the width and height text fields
prevents a small visual bug in Safari where the fields flicker when a
number is typed in.

- Fixes Reset button being too tall on mobile in the paragraph and image
  inspectors
- Makes the margin at bottom of a <ButtonGroup> consistent with all
  other controls (1em)
- Fix regression causing width and height fields from being too close to
  each other

Let the <figure> get its width from the fit-content rule that was added
in 2dc9fb3.

If the user selects a new image, forget any image dimensions that they
added since it likely no longer makes any sense.