[BUGFIX] Fix login redirect_url for fresh logins

The new BE Login form API in commit 9099b64c missed to pass
$_GET['redirect_url'] into the login template, which caused
redirect_url to work only for already logged-in sessions.

Change-Id: I13081f303047e93863d8ce820d8e4fade0cea89b
Resolves: #78719
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/50677


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>