MDL-76432 core_courseformat: migrate file drag and drop

MDL-76432 core_courseformat: migrate file drag and drop

Replace the old course/dragdrop.js file (which was not even minimised)
to AMD modules and integrate them to the new reactive course editor.
From now on, a file can be drop over any course section, no matter if it
is in the course content or in the course index. It will also start
using the new process monitor to show the uploading state to the user.