Allow different locations per product in stock (closes #124)

Kind of basic for now, a different location can be set on purchase, the filters on the stock overview page handles different locations

migrations/0051.sql

@@ -0,0 +1,15 @@
+ALTER TABLE stock
+ADD location_id INTEGER;
+
+ALTER TABLE stock_log
+ADD location_id INTEGER;
+
+CREATE VIEW stock_current_locations
+AS
+SELECT
+	s.product_id,
+	IFNULL(s.location_id, p.location_id) AS location_id
+FROM stock s
+JOIN products p
+	ON s.product_id = p.id
+GROUP BY s.product_id, IFNULL(s.location_id, p.location_id);