Remove references to leanpub book (leanpub pricing changes)

Signed-off-by: David Young <davidy@funkypenguin.co.nz>

manuscript/extras/javascript/feedback.js

@@ -0,0 +1,11 @@
+var feedback = document.forms.feedback
+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)
+})