[!!!][TASK] Migrate EXT:context_help into EXT:backend

Merge the extension into EXT:backend as it only consists of
one AJAX controller - all other functionality related to context
help is already in EXT:backend and deeply connected to Backend-related
output code, so this is just a regular step.

Resolves: #81225
Releases: master
Change-Id: Ie5d577d62876f18414b61f5eefc9d9dd48d52430
Reviewed-on: https://review.typo3.org/52804


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>