Change in file hash results in duplicated images

[emteknetnz]

This PR #518 highlighted a change where manipulated image hashes can be different because of silverstripe/silverstripe-framework#10497

This will result in duplicate images with different files names which is a waste of server storage space

This will affect images directly added to template files with image manipulations performed on them. Off the top of my head I'm not sure if this also affects WYSIWYG inserted images, which presumably would be a larger issue (need to confirm when fixing this work)

A couple of options:

• Create a cleanup task to identify duplicated images
• When performing an manipulation, look for the presence of a duplicate image (regenerate the hash using swapping width/height int with string) - if duplicate found, delete it.