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/providers/edit-provider.html

@@ -1,7 +1,7 @@
 {% extends "withoutnav_template.html" %}
-{% from "components/table.html" import list_table, field, text_field, link_field, right_aligned_field_heading, hidden_field_heading %}
 {% from "components/textbox.html" import textbox %}
 {% from "components/page-footer.html" import page_footer %}
+{% from "components/form.html" import form_wrapper %}
 
 {% block per_page_title %}
 Provider - {{provider.display_name}}
@@ -20,10 +20,10 @@ Provider - {{provider.display_name}}
             <li>We only send from the highest priority provider</li>
         </ul>
 
-        <form method="post">
+        {% call form_wrapper() %}
             {{ textbox(form.priority) }}
             {{ page_footer('Save', back_link=url_for('.view_providers'), back_link_text="Back to providers") }}
-        </form>
+        {% endcall %}
 
     </div>