Better slug in sidenote and header image option in post

src/_includes/sidenote.liquid

@@ -1,3 +1,3 @@
-{% assign sn_slug = sidenote | strip | strip_html | slice: 0, 50 | downcase | replace: '.', '' |replace: ' ', '-' %}{% capture snid %}sn-{{ sn_slug}}{% endcapture %}<label for="{{ snid }}" class="margin-toggle sidenote-number"></label>
+{% assign sn_slug = sidenote | strip | strip_html | strip_newlines | slice: 0, 50 | downcase | remove: '"' | remove: "'" | remove: ' ' | remove: '.' | remove: '-' | remove: '_' %}{% capture snid %}sn-{{ sn_slug}}{% endcapture %}<label for="{{ snid }}" class="margin-toggle sidenote-number"></label>
 <input type="checkbox" id="{{ snid }}" class="margin-toggle">
-<span class="sidenote">{{ sidenote }}</span>
+<span class="sidenote">{{ sidenote | strip_newlines }}</span>

src/_layouts/post.liquid

@@ -29,6 +29,12 @@
     
     <article id="main">
       <section>
+        {% if page.data.header_image %}
+          <figure class="fullwidth">
+            <img style="box-shadow: 1px 6px 14px #ba8" src="{{page.data.header_image}}">
+          </figure>
+        {% endif %}
+
         {{ page.content }}
       </section>
     </article >