tags and special cakes: models, migrations, store layer

Add Tag, SpecialCake, Cake.Tags, Order.SpecialCakes types.
Add migrations for tag, cake_tag, special_cake, special_cake_tag tables.
Add store methods: GetTags, GetCakeTags, SaveCakeTags, CreateTag,
GetSpecialCakes, GetSpecialCakeTags, SaveSpecialCakes, saveSpecialCakeTags.
Load tags in GetCake/GetCakes, cascade tag saves in SaveCake.
Remove debug log calls from ParsePrice.

store/migrations/4.sql

@@ -0,0 +1,5 @@
+create table special_cake_tag (
+    special_cake integer not null references special_cake(id) on delete cascade,
+    tag integer not null references tag(id) on delete cascade,
+    primary key (special_cake, tag)
+);