[BUGFIX] Write config to extTables destroys HTML output

If you enter HTML to update a field in e.g. TCA to write the value into
extTables.php, then the HTML will be printed into the configuration
module.
htmlspecialchar the output.

Fixes: #46999
Releases: 6.1,6.0,4.7,4.5
Change-Id: I390b4252316b8bdf01e5bbcc5a8b33833bdf73e8
Reviewed-on: https://review.typo3.org/19694
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn