fix warning about relationship confilicts

Conflict between Consumable.vehicles and Vehicle.consumables resolved by
making the Consumable.vehicles a read only relationship since we add
consumables only to vehicles. Consumable.vehicles is used in the admin
view to check if a consumable can be deleted safely.

app/entities.py

@@ -164,7 +164,7 @@ class Consumable(db.Model):
     name = db.Column(db.String(255), unique=True)
     unit = db.Column(db.String(255))
 
-    vehicles = db.relationship("Vehicle", secondary=vehicles_consumables)
+    vehicles = db.relationship("Vehicle", secondary=vehicles_consumables, viewonly=True)
 
     def __init__(self, name, unit):
         self.name = name