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/main/views/styleguide.py

@@ -1,7 +1,24 @@
 from flask import render_template
+from flask_wtf import Form
+from wtforms import StringField, PasswordField, TextAreaField, validators
 from app.main import main
 
 
 @main.route('/_styleguide')
 def styleguide():
-    return render_template('views/styleguide.html')
+
+    class FormExamples(Form):
+        username = StringField(u'Username')
+        password = PasswordField(u'Password', [validators.required()])
+        message = TextAreaField(u'Message')
+
+    form = FormExamples()
+
+    form.message.data = "Your vehicle tax for ((registration number)) is due on ((date)). Renew online at www.gov.uk/vehicle-tax"  # noqa
+
+    form.validate()
+
+    return render_template(
+        'views/styleguide.html',
+        form=form
+    )