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">

app/templates/views/platform-admin/returned-letters.html

@@ -1,6 +1,7 @@
 {% extends "views/platform-admin/_base_template.html" %}
 {% 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 %}
   {{ page_title|capitalize }}
@@ -11,10 +12,10 @@
 <div class="grid-row">
   <div class="column-two-thirds">
     <h1 class="heading-large">Submit returned letters</h1>
-    <form method="post" novalidate>
+    {% call form_wrapper() %}
       {{ textbox(form.references, width='1-1', rows=8) }}
       {{ page_footer("Submit") }}
-    </form>
+    {% endcall %}
   </div>
 </div>
 

app/templates/views/platform-admin/services.html

@@ -5,6 +5,7 @@
 {% from "components/big-number.html" import big_number, big_number_with_status %}
 {% from "components/message-count-label.html" import message_count_label %}
 {% from "components/table.html" import mapping_table, field, stats_fields, row_group, row, right_aligned_field_heading, hidden_field_heading, text_field %}
+{% from "components/form.html" import form_wrapper %}
 
 {% macro stats_fields(channel, data) -%}
 
@@ -102,13 +103,13 @@
 
     <details>
       <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") }}
         {{ checkbox(form.include_from_test_key) }}
         </br>
         <button type="submit" class="button">Filter</button>
-      </form>
+      {% endcall %}
     </details>
 
   {% include "views/platform-admin/_global_stats.html" %}