Fix YouTube figures and add a .content-width CSS class

layouts/shortcodes/figures/youtube.html

@@ -4,7 +4,7 @@
 {{- $id := .Get "id" | default (.Get 0) -}}
 {{- $class := .Get "class" | default (.Get 1) -}}
 {{- $title := .Get "title" | default "YouTube Video" }}
-<figure>
+<figure {{ with $class }}class="{{ . }}"{{ end }}>
   <div class="youtube">
     <iframe src="https://{{ $ytHost }}/embed/{{ $id }}{{ with .Get "autoplay" }}{{ if eq . "true" }}?autoplay=1{{ end }}{{ end }}" allowfullscreen frameborder=0 title="{{ $title }}"></iframe>
   </div>