fix(seo) canonical mustn't be relative (#3030)

Author: EugeneHlushko

src/server.jsx

@@ -55,7 +55,7 @@ export default locals => {
             <link key={ path } rel="stylesheet" href={ `/${path}` } />
           ))}
           <link rel="manifest" href="/manifest.json" />
-          <link rel="canonical" href={enforceTrailingSlash(locals.path)} />
+          <link rel="canonical" href={`https://webpack.js.org${enforceTrailingSlash(locals.path)}`} />
           <link rel="apple-touch-icon" href="/images/icons/icon-192x192.png" />
           <link rel="apple-touch-icon" sizes="152x152" href="/images/icons/icon-152x152.png" />
           <link rel="icon" sizes="192x192" href="/images/icons/icon-192x192.png" />