[FEATURE] Create BE-user in installation wizard with a salted password

In the installation wizard (123 steps) the admin user is created with a
md5 hashed password. For security reason this should be changed to a
salted password. As this function is used in #31407 as well it was
refactored to the AbstractAction class.

Resolves: #50612
Releases: 6.2
Change-Id: Ide966eb1c1e05d2190f0d22446bc2b3e92643451
Reviewed-on: https://review.typo3.org/22738
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Stefan Neufeind
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters