[BUGFIX] CSH on popup window - more link leads to non-existent page

Opening CSH help popups from another popup (e.g. add/edit wizard) fails
since the resolved CSH URI is not valid.

Resolves: #80665
Releases: master, 8.7
Change-Id: I16f4dcd037cdb7ab210c083272d0941c979da205
Reviewed-on: https://review.typo3.org/53065


Tested-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Jasmina Ließmann <code@frauliessmann.de>
Reviewed-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>