Add preset for cemetery=sector (#517)

Also added fields for:
* `sector:name`
* a dedicated `ref` field labeled 'Section Number'

data/fields/ref_sector.json

@@ -0,0 +1,5 @@
+{
+    "key": "ref",
+    "type": "text",
+    "label": "Section Number"
+}

data/fields/sector/name.json

@@ -0,0 +1,6 @@
+{
+    "key": "sector:name",
+    "type": "text",
+    "label": "Name",
+    "placeholder": "Dedicated name (if any)"
+}