Use sticky footer on edit template page

If you have a long template it’s annoying to have to scroll all the
way to the bottom to click save, when you’ve only changed a small thing
near the staert of the content.

app/templates/views/edit-sms-template.html

@@ -24,9 +24,11 @@
           {% if current_user.platform_admin %}
             {{ radios(form.process_type) }}
           {% endif %}
-          {{ page_footer(
-            'Save'
-          ) }}
+          <div class="js-stick-at-bottom-when-scrolling">
+            {{ page_footer(
+              'Save'
+            ) }}
+          </div>
         </div>
         <aside class="column-whole">
           {% include "partials/templates/guidance-personalisation.html" %}