[!!!][TASK] Move flash message output to alerts

Since bootstrap is available it would make sense to
use the alert classes instead of the custom implementation.

Resolves: #61959
Releases: master
Change-Id: I0a6723a1bdb7333bc48f71876b8392e78299e4f3
Reviewed-on: http://review.typo3.org/33048
Reviewed-by: Felix Kopp <felix-source@phorax.com>
Tested-by: Felix Kopp <felix-source@phorax.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>