Use a macro for form tags

This will stop us repeatedly forgetting to add `novalidate` and
`autocomplete='off'` to our forms (which is how most of them are set
up).

It uses sensible defaults, based on how we most-commonly configure
forms:
- most of our forms are `post`ed (but this can be overridden)
- `autocomplete` should only be enabled where it makes sense, otherwise
  it’s more annoying than useful (but this can be overriden)
- we should never be using HTML5 form validation because our own error
  styles and messages are better

app/templates/views/api/callbacks/delivery-status-callback.html

@@ -1,6 +1,7 @@
 {% extends "withnav_template.html" %}
 {% from "components/textbox.html" import textbox %}
 {% from "components/page-footer.html" import page_footer %}
+{% from "components/form.html" import form_wrapper %}
 
 {% block service_page_title %}
   Callbacks for delivery receipts
@@ -15,7 +16,7 @@
         Check the <a href="{{ url_for('.callbacks') }}"> callback documentation </a> for more information.
       </p>
 
-      <form method="post">
+      {% call form_wrapper() %}
         {{ textbox(
           form.url,
           width='1-1',
@@ -27,12 +28,12 @@
           hint='At least 10 characters',
           autocomplete='new-password'
         ) }}
-          {{ page_footer(
+        {{ page_footer(
           'Save',
           back_link=url_for(back_link, service_id=current_service.id),
           back_link_text='Back to settings'
-      ) }}
-      </form>
+        ) }}
+      {% endcall %}
     </div>
   </div>