Operator not fully reset after logout

[bastianjoel]

Describe the bug
After logout the operator.service.ts is not completely reset resulting in issues when switching users.
For example in #3965 the Dashboard menu point does not disappear when logging in as anonymous after a user with multiple meetings logged out.
In #3815 another issue is described here: #3815 (comment)

The main problem seems, that _userSubject does not get reset on logout. Other properties which do not get reset and could cause issues are _lastUserId and _lastActiveMeetingId.

Resetting _userSubject will require some adjustments in other places as the user property of the operator service throws an error when accessing it while _userSubject is empty.