[TASK] Streamline EXT:form code

* Register "formvh:" as global namespace, simplifying the templates
* Remove formvh:be.pageRenderer and add the missing files to EXT:fluid
* Migrate callbacks to signal slots and deprecate the callbacks
* Fix plain mail viewhelper, and deprecated it as it can be done in
  Fluid directly
* Code cleanup (fix indention, unused variables etc.)
* Deprecate custom checkbox VH because the fluid bug is gone
* Change tx_form view properties to ".0" instead of ".5" to be in line
  with FSC
* Streamline fluid VH additionalAttributes usage; add compatibility for
  previous configuration; Add BE form editor auto migration
* Fix small JavaScript bugs
* Fix small configuration bugs

Resolves: #80301
Resolves: #80287
Releases: master
Change-Id: Icf68b8a9a6876805af7bf273735d3d30984136ed
Reviewed-on: https://review.typo3.org/51972


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Tested-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>