[FEATURE] Automate user import

To automate user import this patch adds a function to the BE
module for importing users and a Scheduler task for import
automation.

Quite a bit of refactoring happens too, to avoid duplicating
code.

Resolves: #58749
Resolves: #61590
Resolves: #59174
Releases: 1.4
Change-Id: Id1c7a4c67c44f03f3d8848345c58295e77d29d1a
Reviewed-on: http://review.typo3.org/32787
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>