Add a WTForms-compatible textbox macro

This macro:
- accepts a WTForm form field as a parameter
- renders a form field which follows the GOV.UK Elements patterns, both visually
  and in markup terms

It then changes any page which uses either:
- the old, non-WTForms macro or
- the old, WTFforms `render_field` macro

…to use this new macro and removes both of the old ones.

It also adds the option to display hint text above the textbox.

app/templates/views/forgot-password.html

@@ -1,4 +1,6 @@
 {% extends "admin_template.html" %}
+{% from "components/textbox.html" import textbox %}
+{% from "components/page-footer.html" import page_footer %}
 
 {% block page_title %}
 GOV.UK Notify
@@ -12,13 +14,11 @@ GOV.UK Notify
 
     <p>If you have forgotten your password, we can send you an email to create a new password.</p>
 
-    <form autocomplete="off" action="" method="post">
-	  {{ form.hidden_tag() }}
-	  {{ render_field(form.email_address, class='form-control-2-3') }}
-      <p>
-      <button class="button" role="button">Send email</button>
-      </p>
+    <form autocomplete="off" method="post">
+  	  {{ textbox(form.email_address) }}
+      {{ page_footer("Send email") }}
     </form>
+
   </div>
 </div>