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/register.html

@@ -1,4 +1,5 @@
 {% extends "admin_template.html" %}
+{% from "components/page-footer.html" import page_footer %}
 
 {% block page_title %}
 GOV.UK Notify | Create an account
@@ -21,9 +22,7 @@ GOV.UK Notify | Create an account
 			{{ render_field(form.mobile_number, class='form-control-2-3') }}
 			{{ render_field(form.password, class='form-control-2-3') }}
 			<span class="font-xsmall">Your password must have at least 10 characters</span></label>
-			<p>
-			<button class="button" role="button">Continue</button>
-			</p>
+			{{ page_footer("Continue") }}
 		</form>
 	</div>
 </div>