Optimize image storage in save files

[jkoritzinsky]

Currently, once an image is uploaded into a manual, there is no way to remove that image file from the manual save file. Note that users can overwrite that file with an updated image by giving the new image the same file name. We should look into having a solution to remove unused images.

Options:

1. Allow the user to delete images manually from the save file within the editor in some way.
2. Automatically remove unused images from the save file when saving the manual.