[TASK] Separate styling setup from login form creation for BE Login

This change is a pre-patch to re-use the variables of a view in
LoginController, to be used to for custom setups in the extension
configuration.

Resolves: #90468
Releases: master
Change-Id: I7ee0f9467e9a5dce5c419826025314379897ad70
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63347


Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>