Fixed focal point indicator positioning

The focal point indicator is always positioned as a percentage inside
the div.focal-point-chooser block.

For thin images, the image may not exactly fill this block causing the
indicated focal point to be too wide and often not even cover the image.

This commit fixes this by making sure the width and height of the
div.focal-point-chooser block are always the same as the image, even if
the image is too small.
Merge branch 'image-focal-point-chooser-fixes' of https://github.com/kaedroho/wagtail into kaedroho-image-focal-point-chooser-fixes
Release note for #807
Merge branch 'kaedroho-image-focal-point-chooser-fixes' into stable/0.8.x