Added video button to bib entries (alshedivat#2008)

When the video entry is added to bib:

e.g.,

```
{
...
video={https://www.youtube.com/embed/jNQXAC9IVRw}
...
}
```
A dedicated button appears for videos. When clicked, a playable video
appears under the research article (like abstracts and bibtex)

_layouts/bib.html

@@ -164,6 +164,9 @@
             <a href="{{ entry.supp | prepend: '/assets/pdf/' | relative_url }}" class="btn btn-sm z-depth-0" role="button">Supp</a>
             {%- endif %}
           {%- endif %}
+          {%- if entry.video %}
+            <a class="abstract btn btn-sm z-depth-0" role="button">Video</a>
+          {%- endif %}
           {%- if entry.blog %}
             <a href="{{ entry.blog }}" class="btn btn-sm z-depth-0" role="button">Blog</a>
           {%- endif %}
@@ -236,5 +239,13 @@
             {% highlight bibtex %}{{ entry.bibtex | hideCustomBibtex }}{% endhighlight %}
           </div>
           {%- endif %}
+
+          {% if entry.video -%}
+          <!-- Hidden video block -->
+          <div class="abstract hidden">
+            <div style="text-align: center;">{% include video.html path=entry.video class="img-fluid rounded z-depth-1" controls=true %}</div>
+          </div>
+          {%- endif -%}
+
         </div>
       </div>