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/api-keys.html

@@ -1,4 +1,5 @@
 {% extends "withnav_template.html" %}
+{% from "components/page-footer.html" import page_footer %}
 
 {% block page_title %}
 GOV.UK Notify | API keys and documentation
@@ -10,8 +11,9 @@ GOV.UK Notify | API keys and documentation
 
 		<p>Here's where developers can access information about the API and access keys</p>
 
-  	<p><a href="dashboard">Back to dashboard</a></p>
-
-
+		{{ page_footer(
+			back_link=url_for('.dashboard'),
+			back_link_text='Back to dashboard'
+		) }}
 
 {% endblock %}