[BUGFIX] Form padding in Extenson Manager upload

Fixed bottom padding upload form for Extension Manager.

Also back-ports a CSS change into SCSS:
https://review.typo3.org/#/c/34795/

Resolves: #63235
Releases: master
Change-Id: If502837bbe04b1924f91eab6c98d3755a17a5f2c
Reviewed-on: http://review.typo3.org/34796
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>