blocked dates page and warning for special-cake orders

Add blocked_date table (migration 5), model, and store CRUD methods.
Add management page at /blocked with add/delete functionality.
Add nav link 'Terminy zablokowane' with event_busy icon.
Pass blocked dates to order form as JSON data attribute.
On form submit, if the order has special cakes and the date is
blocked, show a confirm() dialog warning the user.

models/data.go

@@ -103,3 +103,8 @@ type CakeCount struct {
 	Name   string
 	Amount int
 }
+
+type BlockedDate struct {
+	ID   int
+	Date string
+}