chore: merge changes from master branch

include/schema/common/head.json

@@ -10,11 +10,6 @@
             "default": "/img/favicon.svg",
             "nullable": true
         },
-        "canonical_url": {
-            "type": "string",
-            "description": "Canonical URL of the current page",
-            "nullable": true
-        },
         "open_graph": {
             "$ref": "/misc/open_graph.json"
         },

layout/common/article.jsx

@@ -8,6 +8,9 @@ const Comment = require('./comment');
  * Get the word count of text.
  */
 function getWordCount(content) {
+    if (typeof content === 'undefined') {
+        return 0;
+    }
     content = content.replace(/<\/?[a-z][^>]*>/gi, '');
     content = content.trim();
     return content ? (content.match(/[\u00ff-\uffff]|[a-zA-Z]+/g) || []).length : 0;