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.
Reduced preview max size on image edit page

This was previously 800 pixels which is too big