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/find-users/find-users-by-email.html

@@ -1,5 +1,6 @@
 {% extends "views/platform-admin/_base_template.html" %}
 {% from "components/page-footer.html" import page_footer %}
+{% from "components/form.html" import form_wrapper %}
 
 {% block per_page_title %}
   Find users by email
@@ -11,11 +12,11 @@
     Find users by email
   </h1>
 
-  <form
-    method="post"
-    action="{{ url_for('.find_users_by_email') }}"
-    class="grid-row"
-  >
+
+  {% call form_wrapper(
+      action=url_for('.find_users_by_email'),
+      class='grid-row'
+  ) %}
     <div class="column-three-quarters">
       {{ textbox(
         form.search,
@@ -27,11 +28,11 @@
       <input type="hidden" name="csrf_token" value="{{ csrf_token() }}">
       <button type="submit" class="button">Search</button>
     </div>
-  </form>
+  {% endcall %}
 
-  <form id="search-form" method="post">
+  {% call form_wrapper(id='search-form' ) %}
     <input type="hidden" name="csrf_token" value="{{ csrf_token() }}">
-  </form>
+  {% endcall %}
 
   {% if users_found %}
   <nav class="browse-list">