Enable feedback widget (#1146)

docs/javascripts/feedback.js

@@ -0,0 +1,13 @@
+var feedback = document.forms.feedback
+feedback.removeAttribute("hidden");
+feedback.addEventListener("submit", function(ev) {
+  ev.preventDefault()
+
+  /* Retrieve page and feedback value */
+  var page = document.location.pathname
+  var data = ev.submitter.getAttribute("data-md-value")
+
+  /* Send feedback value */
+  console.log(page, data)
+  plausible(data)
+})