[BUGFIX] Remove HTML markup from flashMessage

This patch adds the possibility to add line breaks to JS Notifications
to be able to structure the output of extension installation messages.

Resolves: #71518
Releases: master
Change-Id: Id3fffc54f1d47b972cada208c91abe942ca18706
Reviewed-on: https://review.typo3.org/44701


Reviewed-by: Josef Glatz <josef.glatz@typo3.org>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Richard Haeser <richardhaeser@gmail.com>
Tested-by: Richard Haeser <richardhaeser@gmail.com>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>