Reconfigure relationships to squelch sqla warnings

We only ever create new records/relationships by linking up IDs
explicitly (rather than, say, appending them to a relationship), so we
can safely set viewonly=True on most relationsihps.

todosrht/types/label.py

@@ -19,7 +19,8 @@ class Label(Base):
     color = sa.Column(sa.Text, nullable=False)
     text_color = sa.Column(sa.Text, nullable=False)
 
-    tickets = sa.orm.relationship("Ticket", secondary="ticket_label")
+    tickets = sa.orm.relationship("Ticket",
+            secondary="ticket_label", viewonly=True)
 
     __table_args__ = (
         sa.UniqueConstraint("tracker_id", "name",

todosrht/types/ticket.py

@@ -57,14 +57,16 @@ class Ticket(Base):
     anonymous_perms = sa.Column(FlagType(TicketAccess), nullable=True)
     """Permissions granted to anonymous (non-logged in) users"""
 
-    view_list = sa.orm.relationship("TicketSeen")
+    view_list = sa.orm.relationship("TicketSeen", viewonly=True)
 
     labels = sa.orm.relationship("Label",
-        secondary="ticket_label", order_by="Label.name")
+        secondary="ticket_label", order_by="Label.name",
+        viewonly=True)
 
     assigned_users = sa.orm.relationship("User",
         secondary="ticket_assignee",
-        foreign_keys="[TicketAssignee.ticket_id,TicketAssignee.assignee_id]")
+        foreign_keys="[TicketAssignee.ticket_id,TicketAssignee.assignee_id]",
+        viewonly=True)
 
     authenticity = sa.Column(
             sau.ChoiceType(TicketAuthenticity, impl=sa.Integer()),