[BUGFIX] Allow to add custom styles for admin panel

Allow to add custom styles for admin panel, which are included 
below the main styles.

Resolves: #76030
Releases: master, 7.6
Change-Id: If945982a3192f9fca8a008179f4b353eb2e65d76
Reviewed-on: https://review.typo3.org/48061


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Marcin Krzyżanowski <m.krzyzanowski@macopedia.pl>
Tested-by: Marcin Krzyżanowski <m.krzyzanowski@macopedia.pl>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Tested-by: Susanne Moog <typo3@susannemoog.de>