Add ability to show specific concert via a URL parameter

The |concert| parameter can now be used to show a specific concert.

Change-Id: I85ce519296c8a3d5061021f57391290cd4e16520
diff --git a/concerts/nadal2020nens.json b/concerts/nadal2020nens.json
index 9b21169..8bb57db 100644
--- a/concerts/nadal2020nens.json
+++ b/concerts/nadal2020nens.json
@@ -1,4 +1,5 @@
 {
+  "codename": "nadal2020nens",
   "title": "Concert de Nadal 2020",
   "subtitle": "Nens i nenes",
   "begins": 0,