[BUGFIX] Fix display of content area in modal windows in backend

Add scroll possibility for modal body.

Resolves: #81387
Releases: master
Change-Id: I11d54473a6996bee534ea5663cc0de5a4552b66e
Reviewed-on: https://review.typo3.org/52998


Reviewed-by: Markus Sommer <markussom@posteo.de>
Tested-by: Markus Sommer <markussom@posteo.de>
Reviewed-by: Jasmina Ließmann <code@frauliessmann.de>
Tested-by: Jasmina Ließmann <code@frauliessmann.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>