[BUGFIX] Fix panel-collapse and modal-body padding-bottom in firefox

* The last item in panel-collapse has now proper padding to its bottom.
* With a different behaviour of firefox a container having overflow-y
  ignores padding-bottom. A workaround is a pseudo-element reserving
  the same space as the padding attribute would have.

Resolves: #86445
Releases: master
Change-Id: I55b6318f1b59ceda65857726828deab5bc8a306f
Reviewed-on: https://review.typo3.org/58460


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Josef Glatz <josef.glatz@typo3.org>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>