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.

store/migrations/5.sql

@@ -0,0 +1,4 @@
+create table blocked_date (
+    id integer primary key autoincrement,
+    date text not null unique
+);