Improved image size detection for uploaded avatar images

This changeset introduces two fixes that improve the UX for the avatar upload
process with original images of various sizes:

- Be sensitive to the size of the original image when setting the initial
  dimensions of the crop box. If the uploaded image is smaller than the
  "full" avatar size, suggest a crop that contains the entire image; if
  slightly larger than the "full" size, suggest a crop equivalent to the "full"
  size.
- When a too-small image is uploaded, throw a message that suggests a larger
  one.

See #5617

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8673 cdf35c40-ae34-48e0-9cc9-0c9da1808c22