[BUGFIX] Allow to override RTE config via PageTS

Fix the merging of YAML and PageTS provided RTE configuration to allow
editor configuration via PageTS

Resolves: #81880
Releases: master, 8.7
Change-Id: Ic06627686b6ee77d79df34a6dda7b5610ccb06cb
Reviewed-on: https://review.typo3.org/53518


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benjamin Kluge <b.kluge@neusta.de>
Tested-by: Benjamin Kluge <b.kluge@neusta.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>