[BUGFIX] Prevent encoding of HTML entities in backend previews

HTML entities that are used in HTML enabled fields should
be preserved for the preview instead of being encoded (twice).

Change-Id: I4f7b0b7214906f404a1cd1efba009a8cf577a18b
Fixes: #36067
Releases: 6.1
Reviewed-on: https://review.typo3.org/10494
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader