GH-68: Frontera: Cleanup "Full Width" template

- Remove cruft.
- Include custom assets via mini-template (added in 640952b).

frontera-cms/templates/fullwidth.html

@@ -1,22 +1,8 @@
-{% extends "base.html" %}
-{% load cms_tags staticfiles %}
-
-{% block title %}{% page_attribute "page_title" %}{% endblock title %}
-
-{% block assets_font %}
-  {{ block.super }}
-{% endblock assets_font %}
-
-{% block content %}
-{% placeholder "content" %}
-{% endblock content %}
+{% extends "fullwidth.html" %}
+{% load cms_tags %}
 
 {% block assets_custom %}
-  {{ block.super }}
-
-  <!-- To customize styles for this project -->
-  <link rel="stylesheet" href="{% static 'frontera-cms/css/build/site.css' %}">
-  <link rel="stylesheet" href="{% static 'frontera-cms/css/build/site.header.css' %}">
+  {% include "./assets_custom.html" %}
 
   <!-- To style old CMS content on new CMS -->
   <!-- NOTE: This should probably be loaded BEFORE "To customize styles …", but: