Add /_metadata and /_module functions

Also deprecate ?meta and ?module query params.

modules/actions/serveModule.js

@@ -0,0 +1,17 @@
+import serveHTMLModule from './serveHTMLModule';
+import serveJavaScriptModule from './serveJavaScriptModule';
+
+export default function serveModule(req, res) {
+  if (req.entry.contentType === 'application/javascript') {
+    return serveJavaScriptModule(req, res);
+  }
+
+  if (req.entry.contentType === 'text/html') {
+    return serveHTMLModule(req, res);
+  }
+
+  res
+    .status(403)
+    .type('text')
+    .send('module mode is available only for JavaScript and HTML files');
+}