[BUGFIX] EXT:form - mandatoryValidationMessages unset after submit

Build the validation rules within the initializeShowAction

Resolves: #73218
Releases: master, 7.6
Change-Id: I1b441db52da57b8db6a574f87a1600b5332154df
Reviewed-on: https://review.typo3.org/48584


Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Ruud Silvrants <ruudsilvrants@gmail.com>
Tested-by: Ruud Silvrants <ruudsilvrants@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>