[lookandfeel] Fix switching to a different user session

Commit bcaf3886 removed the property `m` in UserDelegate.qml, but it is still used by
`userListCurrentModelData` in SessionManagementScreen.qml at least.

This broke switching to an existing session via the "Switch User" button on the lock screen or the
application launcher/menu, it just hung with an empty screen and this runtime error in the logs:
file:///usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/lockscreen/LockScreenUi.qml:444: TypeError: Cannot read property 'vtNumber' of undefined

Adding it back makes switching work again (and gets rid of the runtime error).

CCBUG: 423526
FIXED-IN: 5.20.3