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/organisations/organisation/users/invite-org-user.html

@@ -2,6 +2,7 @@
 
 {% 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 %}
   Invite a team member
@@ -13,7 +14,7 @@
     Invite a team member
   </h1>
   <div class="grid-row">
-	  <form method="post" class="column-three-quarters" novalidate>
+    {% call form_wrapper(class="column-three-quarters") %}
 	    {{ textbox(form.email_address, width='1-1', safe_error_message=True) }}
 
 	    <div class="bottom-gutter">
@@ -27,7 +28,7 @@
 			</div>
 
 	    {{ page_footer('Send invitation email') }}
-	  </form>
+	  {% endcall %}
   </div>
 
 {% endblock %}

app/templates/views/organisations/organisation/users/user/index.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 %}
   {{ user.name or user.email_localpart }}
@@ -15,7 +16,8 @@
   <p>
     {{ user.email_address }}
   </p>
-  <form method="post" class="column-three-quarters">
+
+  {% call form_wrapper(class="column-three-quarters") %}
       {{ page_footer(
         'Save',
         back_link=url_for('.manage_org_users', org_id=current_org.id),
@@ -23,6 +25,6 @@
         delete_link=url_for('.remove_user_from_organisation', org_id=current_org.id, user_id=user.id) if user or None,
         delete_link_text='Remove user from organisation'
       ) }}
-  </form>
+  {% endcall %}
 
 {% endblock %}