[TASK] Streamline usage and design of FlashMessages in the BE

This patch streamline the usage and design of FlashMessages in the BE.
The markup is now centralized in a method of the FlashMessage class.
In all places where FlashMessages rendered, this method is called.

Resolves: #73698
Releases: master
Change-Id: I5ef18a95ea15949e8cede71227101f2cc1ce30d3
Reviewed-on: https://review.typo3.org/46927


Reviewed-by: Benjamin Kott <info@bk2k.info>
Tested-by: Benjamin Kott <info@bk2k.info>
Reviewed-by: Timo Schmidt <timo-schmidt@gmx.net>
Tested-by: Timo Schmidt <timo-schmidt@gmx.net>