[BUGFIX] Move beuser property mappings to global scope

Currently the persistence settings for all tx_beuser models are defined
in module.tx_beuser.persistence.classes.
If any other module tries to access the corresponding backend user
repository from tx_beuser, then no mappings will be loaded by extbase,
resulting in a wrong cache entry for the model and the attempt to
access a nonexisting table.

Move the class mapping part of the TS template into
  config.tx_extbase.persistence.classes
to make them available for all extensions and modules.

Resolves: #51234
Releases: 6.2, 6.1, 6.0
Change-Id: I0b39c96c9db89c379e5613231a215f27b2dfbcde
Reviewed-on: https://review.typo3.org/23967
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind