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/new-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
@@ -13,17 +15,9 @@ GOV.UK Notify
 
 		<p> You can now create a new password for your account.</p>
 
-		<form action="" autocomplete="off" method="post">
-			{{ form.hidden_tag() }}
-		<p>
-  		  {{ render_field(form.new_password, class="form-control-1-4", type="password") }}
-  		<span class="font-xsmall">Your password must have at least 10 characters</span>
-  		</p>
-
-  		<p>
-  		<button class="button" role="button">Continue</button>
-  		</p>
-
+		<form method="post" autocomplete="off">
+		  {{ textbox(form.new_password, hint="Your password must have at least 10 characters") }}
+  		{{ page_footer("Continue") }}
 		</form>
 		{% else %}
 			Message about email address does not exist. Some one needs to figure out the words here.