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/text-not-received.html

@@ -1,4 +1,5 @@
 {% extends "admin_template.html" %}
+{% from "components/textbox.html" import textbox %}
 {% from "components/page-footer.html" import page_footer %}
 
 {% block page_title %}
@@ -13,9 +14,8 @@ GOV.UK Notify
 
     <p>Check your mobile phone number is correct and then resend the confirmation code.</p>
 
-      <form autocomplete="off" action="" method="post">
-        {{ form.hidden_tag() }}
-        {{ render_field(form.mobile_number, class='form-control-2-3') }}
+      <form autocomplete="off" method="post">
+        {{ textbox(form.mobile_number) }}
         {{ page_footer("Resend confirmation code") }}
       </form>
   </div>