Add basic flow for adding email _or_ sms templates

Templates now have:
- a type (email or sms)
- a subject (if they are email templates)

We don’t want two completely separate view files for email and SMS, because they
would have an enormous amount of repetition.

So this commit adds
- different templates for SMS and email templates
- different form objects for SMS and email templates

…and wires them up.

app/templates/views/edit-email-template.html

@@ -0,0 +1,43 @@
+{% extends "withnav_template.html" %}
+{% from "components/textbox.html" import textbox %}
+{% from "components/page-footer.html" import page_footer %}
+
+{% block page_title %}
+  {{ h1 }} – GOV.UK Notify
+{% endblock %}
+
+{% block maincolumn_content %}
+
+    <h1 class="heading-large">Edit email template</h1>
+
+    <form method="post">
+      <div class="grid-row">
+        <div class="column-two-thirds">
+          {{ textbox(form.name, width='1-1') }}
+          {% if 'email' == template_type %}
+            {{ textbox(form.subject, width='1-1') }}
+          {% endif %}
+        </div>
+      </div>
+      <div class="grid-row">
+        <div class="column-two-thirds">
+          {{ textbox(form.template_content, highlight_tags=True, width='1-1') }}
+        </div>
+        <div class="column-one-third">
+          <label for='template_content' class='edit-template-placeholder-hint'>
+            Add placeholders using double brackets, eg Your thing
+            is due on ((date))
+          </label>
+        </div>
+      </div>
+      {{ page_footer(
+        'Save',
+        delete_link=url_for('.delete_service_template', service_id=service_id, template_id=template_id) if template_id or None,
+        delete_link_text='Delete this template',
+        back_link=url_for('.choose_template', template_type=template_type, service_id=service_id),
+        back_link_text='Cancel'
+      ) }}
+    </form>
+
+
+{% endblock %}