[TASK] Move JavaScript files to EXT: backend

This patch moves few JavaScript files to EXT: backend.

Still few JavaScript files remain in /typo3 and /js folder.
Those JavaScript files are used within the backend and
therefore should be moved to EXT: backend.

Change-Id: I62b9861d66f219f34e53afa5dd7a1d9c20073a98
Resolves: #49507
Releases: 6.2
Reviewed-on: https://review.typo3.org/21721
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind