Add loading image behind slides

assets/sass/components/_all.sass

@@ -1,6 +1,7 @@
 @import 'availability'
 @import 'badges'
 @import 'content-types/blog-post'
+@import 'content-types/talk'
 @import 'footer'
 @import 'meetups'
 @import 'testimonials'

assets/sass/components/content-types/_talk.sass

@@ -0,0 +1,6 @@
+.presentation
+    background: url('../images/loading.gif') no-repeat center
+    min-height: 350px
+    width: 100%
+    +desktop
+        min-height: 550px

source/_partials/talk/slides.twig

@@ -1,6 +1,8 @@
 {% if page.slides_embed %}
     <div class="talk-slides">
         <h2>Slides</h2>
+        <div class="presentation">
             {{ page.slides_embed|raw }}
         </div>
+    </div>
 {% endif %}