[BUGFIX] Hash identifier of Flexform sections

To avoid issues with Flexform section identifiers having any reserved
character in CSS context, e.g. a dot, the identifier is now hashed to
be able to find the container via `querySelector()`.

Resolves: #94541
Releases: master
Change-Id: Id2a63ae679e024835284672b2121af9003e58f51
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69815


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>