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/templates.py

@@ -1,6 +1,7 @@
 from flask import request, render_template, redirect, url_for
 
 from app.main import main
+from app.main.forms import TemplateForm
 
 
 @main.route("/templates")
@@ -10,12 +11,17 @@ def manage_templates():
 
 @main.route("/templates/template", methods=['GET', 'POST'])
 def add_template():
+
+    form = TemplateForm()
+
+    form.template_name.data = 'Reminder'
+    form.template_body.data = 'Vehicle tax: Your vehicle tax for ((registration number)) expires on ((date)). Tax your vehicle at www.gov.uk/vehicle-tax'  # noqa
+
     if request.method == 'GET':
         return render_template(
             'views/edit-template.html',
-            template_name='Reminder',
-            template_body='Vehicle tax: Your vehicle tax for ((registration number)) expires on ((date)). Tax your vehicle at www.gov.uk/vehicle-tax',  # noqa
-            h1='Edit template'
+            h1='Edit template',
+            form=form
         )
     elif request.method == 'POST':
         return redirect(url_for('.manage_templates'))
@@ -23,10 +29,14 @@ def add_template():
 
 @main.route("/templates/template/add", methods=['GET', 'POST'])
 def edit_template():
+
+    form = TemplateForm()
+
     if request.method == 'GET':
         return render_template(
             'views/edit-template.html',
-            h1='Add template'
+            h1='Add template',
+            form=form
         )
     elif request.method == 'POST':
         return redirect(url_for('.manage_templates'))