[BUGIFX] Escape backslashes in JSON storage

Any backslashes in the mappings configuration must be escaped
before storage as JSON, otherwise they will be considered as
special characters and break the JSON format (or simply ignored
if not corresponding to special sequences).

Resolves: #58309
Releases: 1.7
Change-Id: I7f744a9f6e5e0c26e035d74b869917a5b652d168
Reviewed-on: https://review.typo3.org/29900
Reviewed-by: Francois Suter
Tested-by: Francois Suter