[BUGFIX] Quickedit mode: RTE has no user CSS

A similar issue arises with configured microdata schema.
This is a follow-up to change https://review.typo3.org/#/c/33569/

Releases: master, 6.2
Resolves: #56408
Change-Id: Ic8122d7a40372f1da85b31d1bd3a7677f2c14a2b
Reviewed-on: http://review.typo3.org/33823
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>