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/email-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
@@ -18,8 +19,8 @@ GOV.UK Notify
           {{ render_field(form.email_address, class='form-control-2-3') }}
           <span class="font-xsmall">Your email address must end in .gov.uk</span>
           <p>
-          <button class="button" role="button">Resend confirmation code</button>
           </p>
+          {{ page_footer('Resend confirmation code') }}
       </form>
   </div>
 </div>