[TASK] Move js files to EXT: backend

The global js/ directory should be resolved. Moves several files
to EXT: backend that are only used in EXT: backend classes.

Change-Id: I79c80f60699750a5aff46ac554cb8d92c8a37236
Resolves: #48063
Releases: 6.2
Reviewed-on: https://review.typo3.org/20644
Reviewed-by: Thomas Maroschik
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn