Add missing cover field to note blueprint and move field to its own blueprint

site/blueprints/fields/cover.yml

@@ -0,0 +1,9 @@
+# Fields can be defined in there own field blueprints
+# and reused across multiple blueprints
+# This field is used in the album.yml and note.yml blueprints
+# See http://getkirby.test/docs/guide/blueprints/extending-blueprints#reusing-and-extending-single-fields
+type: files
+multiple: false
+query: page.images.template('image')
+uploads:
+  template: image