Add reset_image_ids from shish/shimmie2-utils to the admin extension

I've added the code from the reset_image_ids thing in the utils repo to the admin extension.

I've also moved the checkbox from the before the button to after the button for protected items, so that it's consistent with the placement of other checkboxes in the UI (especially the main Config page). That... could be a config option, I don't know.