[BUGFIX] FlashMessages container interrupts the backend usage

If flash messages are displayed, the underlaying elements in the
backend are not clickable anymore due to improper CSS.
This patch changes the positioning of the container that holds
the flash messages. Please see the screenshot in #63323 for
a visualization of the problem.

Resolves: #63323
Releases: master
Change-Id: I80b7debddd556bb3d2a0614049bc0c8137f3fd87
Reviewed-on: http://review.typo3.org/34607
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>