Use jQuery slim

Beautiful Jekyll from which this theme was ported switched to the slim version,
which doesn't include the `$.ajax()` function.  I changed the Staticman JS
script in the same way as my PR daattali/beautiful-jekyll#782 to make it
compatible with the slim version.

layouts/partials/footer.html

@@ -63,12 +63,12 @@
 {{- if .Site.Params.selfHosted -}}
 <script src="{{ "js/katex.min.js" | absURL }}"></script>
 <script src="{{ "js/auto-render.min.js" | absURL }}"></script>
-<script src="{{ "js/jquery.min.js" | absURL }}"></script>
+<script src="{{ "js/jquery-3.5.1.slim.min.js" | absURL }}"></script>
 <script src="{{ "js/bootstrap.min.js" | absURL }}"></script>
 {{- else -}}
 <script src="https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.10.0/katex.min.js" integrity="sha384-K3vbOmF2BtaVai+Qk37uypf7VrgBubhQreNQe9aGsz9lB63dIFiQVlJbr92dw2Lx" crossorigin="anonymous"></script>
 <script src="https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.10.0/contrib/auto-render.min.js" integrity="sha384-kmZOZB5ObwgQnS/DuDg6TScgOiWWBiVt0plIRkZCmE6rDZGrEOQeHM5PcHi+nyqe" crossorigin="anonymous"></script>
-<script src="https://code.jquery.com/jquery-1.12.4.min.js" integrity="sha256-ZosEbRLbNQzLpnKIkEdrPv7lOy9C27hHQ+Xp8a4MxAQ=" crossorigin="anonymous"></script>
+<script src="https://code.jquery.com/jquery-3.5.1.slim.min.js"></script>
 <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>
 {{- end }}
 

static/js/staticman.js

@@ -5,24 +5,30 @@
 
   $('.js-form').submit(function () {
     var form = this;
+    let url = $(this).attr('action');
+    let data = $(this).serialize();
 
     $(form).addClass('form--loading');
 
-    $.ajax({
-      type: $(this).attr('method'),
-      url: $(this).attr('action'),
-      data: $(this).serialize(),
-      contentType: 'application/x-www-form-urlencoded',
-      success: function (data) {
-        showModal('Perfect !', 'Thanks for your comment! It will show on the site once it has been approved. .');
-        $(form).removeClass('form--loading');
-      },
-      error: function (err) {
-        console.log(err);
-        showModal('Error', 'Sorry, there was an error with the submission!');
-        $(form).removeClass('form--loading');
+    var xhr = new XMLHttpRequest();
+    xhr.open("POST", url);
+    xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
+    xhr.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
+    xhr.onreadystatechange = function () {
+      if(xhr.readyState === XMLHttpRequest.DONE) {
+        var status = xhr.status;
+        if (status >= 200 && status < 400) {
+          showModal('Perfect !', 'Thanks for your comment! It will show on the site once it has been approved. .');
+          $(form).removeClass('form--loading');
+        } else {
+          console.error(xhr.statusText);
+          showModal('Error', 'Sorry, there was an error with the submission!');
+          $(form).removeClass('form--loading');
+        }
       }
-    });
+    };
+
+    xhr.send(data);
 
     return false;
   });