[BUGFIX] Missing encoding in flexforms IRRE javascript

encodeURIComponent is added to escape all special characters in
parameters for AJAX call.

Resolves: #54304
Releases: 6.2, 6.1, 6.0
Change-Id: I3559104e1a26241b519f40a10000637852a4f114
Reviewed-on: https://review.typo3.org/26283
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind