clean up primary_heroguts.html a little

[Pomax]

we did some cleanup in the mozfest app to yield:

 <div class="banner">
    {% if page.banner %}
      {% if homepage %}
        {% include "./homepage_banner.html" with page=page %}
        {% include "../fragments/hero/large.html" with page=page %}
      {% else %}
        {% include "../fragments/hero/normal.html" with page=page %}
      {% endif %}
    {% else %}
    <img src="/_images/mozfest/Festival Banner 1.jpg" alt="">
    {% endif %}
  </div>

rather than a huge block of HTML, with the includes in their own nice-and-terse files. That's worth doing for the foundation site itself, too

[Pomax]

I'll see if this is still something we need to do.