Fix email names

templates/email/email_assigned_production_officer.html

 <p>
-    Dear {{ stream.invitations_officer.user.last_name }},
+    Dear {{ stream.officer.user.last_name }},
 </p>
 <p>
     You are now assigned as Production Officer to the stream

templates/email/email_assigned_production_officer.txt

-Dear {{ stream.invitations_officer.user.last_name }},
+Dear {{ stream.officer.user.last_name }},
 \n
 \n
 You are now assigned as Production Officer to the stream

templates/email/email_assigned_supervisor.html

 <p>
-    Dear {{ stream.invitations_officer.user.last_name }},
+    Dear {{ stream.supervisor.user.last_name }},
 </p>
 <p>
     You are now assigned as Production Supervisor to the stream

templates/email/email_assigned_supervisor.txt

-Dear {{ stream.invitations_officer.user.last_name }},
+Dear {{ stream.supervisor.user.last_name }},
 \n
 \n
 You are now assigned as Production Supervisor to the stream