Replace Darkroom.js with another library

[benfes]

It looks like the H5P editor uses Darkroom.js for image editing, the version included in the codebase is out of date. However, darkroom.js no longer appears to be maintained. See: https://github.com/MattKetmo/darkroomjs

Perhaps this should be updated to another library such as https://www.ktm.sh/pintura which darkroom.js suggests on their github page.