[BUGFIX] Flexform section container expand / collapse

Change-Id: Ie35ced0d902497a6eacf2d3e9a8463cdd2066ac8
Resolves: #71183
Releases: master
Reviewed-on: https://review.typo3.org/47934


Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Benjamin Kott <benjamin.kott@outlook.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>