Add memorial theme, some fixes to benches and artwork to support memorial theme

assets/themes/memorials/memorials.json

@@ -0,0 +1,29 @@
+{
+  "id": "memorials",
+  "icon": "./assets/layers/memorial/memorial.svg",
+  "description": {
+    "en": "Memorials are physical objects permantently placed in the public space to remember a person or event. They can be a wide range of objects, such as statues, plaques, paintings, military objects (such as tanks), ..."
+  },
+  "title": {
+    "en": "Memorials"
+  },
+  "layers": [
+    {
+      "builtin": [
+        "ghost_bike",
+        "memorial"
+      ],
+      "override": {
+        "minzoom": 9
+      }
+    },
+    {
+      "builtin": [
+        "bench"
+      ],
+      "override": {
+        "minzoom": 18
+      }
+    }
+  ]
+}