404 page style (#910)

* RTD activated custom 404 a few days ago and our custom 404 broke

This custom template was never tested on RTD before as it was not taken in
account but now that it is, we can see it fails.

* update the url to use config

* removed issue tracker link as we will change it soon.

Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

docs/custom_theme/404.html

@@ -1,10 +1,15 @@
-{% extends "base.html" %}
+{% extends "main.html" %}
 {% block content %}
 
 <p style="text-align: center;font-size: 3em"><i class="md-icon">account_balance</i></p>
 <h1 id="404-page-not-found" style="text-align: center">404</h1>
 <p style="text-align: center"><strong>Deity not found in the Pantheon</strong></p>
-<p style="text-align: center">Try the <a href="/">homepage</a>, or use the search field on the top right of this documentation.</p>
-<p style="text-align: center">If you think we made a mistake and deleted a page that should be here, then please tell us on <a href="https://gitter.im/PegaSysEng/pantheon">Gitter</a> or create an issue in <a href="{{ config.repo_url + '/issues' | url }}">Pantheon Github repository</a>.</p>
+<p style="text-align: center">
+  Try the <a href="/">homepage</a>, or use the search field on the top right of this documentation.
+</p>
+<p style="text-align: center">
+  If you think we made a mistake and deleted a page that should be here, then please let us know on
+  <a href="{{ config.extra.support.gitter | url }}">Gitter</a>.
+</p>
 
 {% endblock %}