[BUGFIX] Hide CSH button if context_help is disabled

Do not show the help icon if the extension "context_help" is not installed.

Change-Id: Ia23c5c1a2d61a1e24e9de96def2fdb6a0a69b99f
Resolves: #71659
Releases: master
Reviewed-on: https://review.typo3.org/45197


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>