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/platform-admin/index.html

@@ -3,6 +3,7 @@
 {% from "components/big-number.html" import big_number_simple %}
 {% from "components/message-count-label.html" import message_count_label %}
 {% from "components/status-box.html" import status_box %}
+{% from "components/form.html" import form_wrapper %}
 
 {% block per_page_title %}
   Platform admin
@@ -15,12 +16,12 @@
   </h1>
   <details {% if form.errors %}open{% endif %}>
     <summary>Apply filters</summary>
-    <form autocomplete="off" method="get">
+    {% call form_wrapper(method="get") %}
       {{ textbox(form.start_date, hint="Enter start date in format YYYY-MM-DD") }}
       {{ textbox(form.end_date, hint="Enter end date in format YYYY-MM-DD") }}
       </br>
       <button type="submit" class="button">Filter</button>
-    </form>
+    {% endcall %}
   </details>
 
   <div class="grid-row bottom-gutter">