Fix file permission handling in BackendUserAuthentication
Take default TSConfig file permissions into account:
File permissions configured in User TSConfig,
are not taken into account, because the properties
from the getTSConfig method are returned in a
'properties' key of an array but the top level
array is used. Use getTSConfigProp instead.
Fix the bit wise check for old file permissions:
XOR the value is wrong because it gives the wrong
result if more bits are set. Use AND operation
to properly check if a specific bit is not set.
Check if the user is admin in getFilePermissionsForStorage()
Add tests to confirm the desired behaviour.