Rename submit_form component to page_footer

Submit form was
- a confusing name in itself
- not descriptive, because it also has an optional ‘back’ link

This commit also puts this component in as many pages as possible, stripping
out any hard-coded buttons and links.

It replaces any usage of buttons for ‘back’, because these are links, not
buttons (ie they don’t change any data).

app/templates/admin_template.html

@@ -20,11 +20,9 @@
   GOV.UK Notify admin
 {% endblock %}
 
-
 {% block cookie_message %}
 {% endblock %}
 
-
 {% block inside_header %}
 {% endblock %}
 {% block header_class %}with-proposition{% endblock %}