Add anonymous upvote functionality

layouts/_default/single.html

@@ -18,6 +18,15 @@
     <a class="blog-tags" href="{{ .RelPermalink }}">#{{ lower .LinkTitle }}</a>
   {{ end }}
 </p>
+{{ if .Site.Params.upvotes }}
+<div class="upvote">
+  <button id="upvote-btn" aria-label="Upvote">
+    <span id="upvote-chevron">&#8963;</span>
+    <span id="upvote-count">—</span>
+  </button>
+</div>
+<script src="/upvote.js"></script>
+{{ end }}
 {{ if not .Params.hideReply }}
 {{ with .Site.Params.author.email }}
   <p>