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.

server/templates/helpers.go

@@ -1,6 +1,7 @@
 package templates
 
 import (
+	"encoding/json"
 	"fmt"
 	"strconv"
 	"strings"
@@ -73,6 +74,15 @@ func orderItemsDesc(order models.Order) string {
 	return desc
 }
 
+func blockedDatesJSON(dates []models.BlockedDate) string {
+	names := make([]string, len(dates))
+	for i, d := range dates {
+		names[i] = d.Date
+	}
+	b, _ := json.Marshal(names)
+	return string(b)
+}
+
 func orderScript(order models.Order) string {
 	var b strings.Builder
 	b.WriteString("<script>")