Themes(public_notice_boards): Fix issues with title

This theme is about boards where the public can freely add posters for
for example local events. This theme was created in response to a
question by someone from a cultural organization who is interested in
mapping and seeing all the public notice boards in Ghent. There is
overlap with the existing advertising theme, but this theme has a very
different focus.

Wikipedia page (fr): https://fr.wikipedia.org/wiki/Affichage_libre
Info page about the boards in Ghent (nl): https://stad.gent/nl/student-in-gent/studenten-vrije-tijd/fuiven-en-evenementen-organiseren/affiches-plakken

The tags of interest are `advertising=board` and `access=yes`. A
particular subtag is `message=*`, this theme only shows `showbiz`,
`non_profit`, `opinion`, `religion`, since other values are seldom or
never applicable for public notice boards.

Since many public notice boards are still mapped without `access=yes`,
at high zoom the theme shows all `advertising=board`s, using the
override technique described in Making_Your_Own_Theme.md section
"Don't: use a layer to filter".

assets/layers/public_notice_boards/public_notice_boards.json

@@ -19,7 +19,8 @@
   "minzoom": 12,
   "title": {
     "render": {
-      "*": "Public notice board"
+      "en": "Public notice board",
+      "nl": "Affichebord voor vrij aanplakken"
     }
   },
   "pointRendering": [

assets/themes/public_notice_boards/public_notice_boards.json

@@ -45,6 +45,10 @@
             ]
           }
         },
+        "title": {
+          "en": "Advertising board",
+          "nl": "Affichebord"
+        },
         "+tagRenderings": [
           {
             "id": "advertising_board-access",