Add a preview of basic view

One of the big things we found in user research was that people were
uncertain what the effect of giving someone basic view was.

So in the spirit of ‘show don’t tell’, this commit adds a way for users
to preview basic view. They can go into the preview and click around as
much as they like, just as if they really had the basic view assigned to
them.

Once they have seen enough they can return to the settings page where
they can decide whether or not to switch basic view on for real.

app/templates/error/403.html

@@ -1,5 +1,5 @@
 {% extends "withoutnav_template.html" %}
-{% block per_page_title %}Forbidden{% endblock %}
+{% block per_page_title %}403{% endblock %}
 {% block maincolumn_content %}
     <div class="grid-row">
       <div class="column-two-thirds">