[TASK] Move admin_panel.css to EXT:frontend

The admin panel references t3skin to load the CSS file,
which should belong to EXT:frontend though.

Resolves: #72638
Releases: master
Change-Id: Ibd38c8d53134eb02d611ce88dc0f5eb7914638e8
Reviewed-on: https://review.typo3.org/45816


Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Tested-by: Susanne Moog <typo3@susannemoog.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>