middleware: Use request.user for logging when possible.

Instead of trying to set the _requestor_for_logs attribute in all the
relevant places, we try to use request.user when possible (that will be
when it's a UserProfile or RemoteZulipServer as of now). In other
places, we set _requestor_for_logs to avoid manually editing the
request.user attribute, as it should mostly be left for Django to manage
it.
In places where we remove the "request._requestor_for_logs = ..." line,
it is clearly implied by the previous code (or the current surrounding
code) that request.user is of the correct type.

zerver/lib/test_helpers.py

@@ -280,7 +280,6 @@ class HostRequestMock:
         self.method = ''
         self.body = ''
         self.content_type = ''
-        self._requestor_for_logs = ''
 
     def get_host(self) -> str:
         return self.host