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/blocked_date.go

@@ -0,0 +1,33 @@
+package store
+
+import (
+	"git.bronku.xyz/bronku/cake-order-tracker/models"
+)
+
+func (s *Store) GetBlockedDates() ([]models.BlockedDate, error) {
+	rows, err := s.db.Query("select id, date from blocked_date order by date;")
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var out []models.BlockedDate
+	for rows.Next() {
+		var b models.BlockedDate
+		if err := rows.Scan(&b.ID, &b.Date); err != nil {
+			return nil, err
+		}
+		out = append(out, b)
+	}
+	return out, rows.Err()
+}
+
+func (s *Store) AddBlockedDate(date string) error {
+	_, err := s.db.Exec("insert or ignore into blocked_date(date) values (?);", date)
+	return err
+}
+
+func (s *Store) RemoveBlockedDate(id int) error {
+	_, err := s.db.Exec("delete from blocked_date where id = ?;", id)
+	return err
+}

store/migrations/5.sql

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