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/send-email.html

@@ -6,7 +6,6 @@ GOV.UK Notify | Send email
 
 {% block maincolumn_content %}
 
-
 		<h1 class="heading-xlarge">Send email</h1>
 
 		<p>This page will be where we construct email messages</p>
@@ -15,5 +14,4 @@ GOV.UK Notify | Send email
   	<a class="button" href="check-email" role="button">Continue</a>
   	</p>
 
-
 {% endblock %}