[BUGFIX] Fix maximum height of preview links dialog

If there are many previewable languages on one page the modal dialog
now will have a scroll bar for more than 4 entries, so it doesn't
break the UI any more.

Add 'modal-inner-scroll' class to modal 'show' call to let the
JS framework handle the maximum height / scroll behavior.

Resolves: #69637
Releases: master, 7.6
Change-Id: I59f91c4364d0d611c5822492b9ace513b1a45a33
Reviewed-on: https://review.typo3.org/49511


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Gianluigi Martino <gmartino27@gmail.com>
Tested-by: Gianluigi Martino <gmartino27@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>