[BUGFIX] Frontend shows HTML tags in messages

Hardcoded linebreak tags in StatusMessages were not resolved to
linebreaks, but included into the output.
Replacing the <br /> tags with the nl command using the nl2br
viewhelper on the message output solves the problem.

Releases: 6.2
Resolves: #49266
Change-Id: I07f6dd389df1ce3eedb2b2f957da91f14b8f566f
Reviewed-on: https://review.typo3.org/21551
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters