[BUGFIX] RTE: Required ExtJs css files are not loaded in frontend

Solution: Add ExtJs css files required by the RTE

Resolves: #56027
Releases: 6.2
Change-Id: I56c81a0119a34097aa8c4a3dd15655cb5a6ca635
Reviewed-on: https://review.typo3.org/27647
Reviewed-by: Wouter Wolters
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland