Use id attribute to query reading progress element (#467)

no issue
- using an `id` selector doesn't break if other `progress` elements are present on a page

partials/floating-header.hbs

@@ -20,7 +20,7 @@
             {{> "icons/facebook"}}
         </a>
     </div>
-    <progress class="progress" value="0">
+    <progress id="reading-progress" class="progress" value="0">
         <div class="progress-container">
             <span class="progress-bar"></span>
         </div>

post.hbs

@@ -143,7 +143,7 @@ $(document).ready(function () {
     $postContent.fitVids();
     // End fitVids
 
-    var progressBar = document.querySelector('progress');
+    var progressBar = document.querySelector('#reading-progress');
     var header = document.querySelector('.floating-header');
     var title = document.querySelector('.post-full-title');