[TASK] EXT:form - Optimize frontend templates

* add attribute role="alert" to error hints
* streamline bootstrap 3 error classes
* add attribute formnovalidate to previous button of multistep form
* remove fieldset wrap from element type 'PAGE'

Resolves: #69974
Releases: master
Change-Id: I66336879bed0f94752482404c3ed7cfa7028046d
Reviewed-on: https://review.typo3.org/51426


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>