[TASK] Harmonize allowed characters of backend usernames

When creating new backend users from the intall tool, any
character is now accepted for the username. Whitespaces get
trimmed and uppercase is converted to lowercase.
This is already the case for TCA of be_users.username

Change-Id: I0edabd2cb82dc59bdc52dd385c1e198f04be6017
Resolves: #50875
Releases: master
Reviewed-on: http://review.typo3.org/35997


Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>