[BUGFIX] Flexform Form ID should not contain brackets

Using brackets in flexform elements caused the javascript to be disabled
for the element.
While using a custom form id that can be used as HTML id="" this is
bypassed and fixes all javascript interaction.

Releases: master, 8.7
Resolves: #80908
Change-Id: I91f39b2fed61076b7139603fedc7b268546d64cd
Reviewed-on: https://review.typo3.org/52771


Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>