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/views/service-settings/delete.html

@@ -1,5 +1,5 @@
 {% extends "withnav_template.html" %}
-{% from "components/submit-form.html" import submit_form %}
+{% from "components/page-footer.html" import page_footer %}
 
 {% block page_title %}
   GOV.UK Notify | Service settings
@@ -31,7 +31,7 @@
       </ul>
 
       <form method="post">
-        {{ submit_form(
+        {{ page_footer(
           'Yes, delete ‘{}’'.format(service.name),
           destructive=True,
           back_link=url_for('.service_settings')