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/text-not-received.html

@@ -1,4 +1,5 @@
 {% extends "admin_template.html" %}
+{% from "components/page-footer.html" import page_footer %}
 
 {% block page_title %}
 GOV.UK Notify
@@ -15,9 +16,7 @@ GOV.UK Notify
       <form autocomplete="off" action="" method="post">
         {{ form.hidden_tag() }}
         {{ render_field(form.mobile_number, class='form-control-2-3') }}
-        <p>
-        <button class="button" role="button">Resend confirmation code</button>
-        </p>
+        {{ page_footer("Resend confirmation code") }}
       </form>
   </div>
 </div>