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/two-factor.html

@@ -1,4 +1,5 @@
 {% extends "admin_template.html" %}
+{% from "components/page-footer.html" import page_footer %}
 
 {% block page_title %}
 GOV.UK Notify | Text verification
@@ -16,10 +17,8 @@ GOV.UK Notify | Text verification
 		<form autocomplete="off" action="" method="post">
 			{{ form.hidden_tag() }}
 			{{ render_field(form.sms_code, class='form-control-1-4') }}
-  			<span class="font-xsmall"><a href="{{ url_for('.verification_code_not_received') }}">I haven't received a text</a></span>
-  			<p>
-  			<button class="button" role="button">Continue</button>
-  			</p>
+			<span class="font-xsmall"><a href="{{ url_for('.verification_code_not_received') }}">I haven't received a text</a></span>
+			{{ page_footer("Continue") }}
 		</form>
 	</div>
 </div>