Stop users from editing/adding templates without the correct permission

If sending SMS is disabled for a service, it should not be possible to
add or modify SMS templates. If a user tries to do this, they should see
a different page with a link to go back. The same thing should happen
with email templates.

app/templates/views/templates/action_blocked.html

@@ -0,0 +1,47 @@
+{% extends "withnav_template.html" %}
+{% from "components/textbox.html" import textbox %}
+{% from "components/page-footer.html" import page_footer %}
+
+{% block service_page_title %}
+  Emails
+{% endblock %}
+
+{% block maincolumn_content %}
+
+  <div class="grid-row">
+    <div class="column-five-sixths">
+      <h1 class="heading-large">{{ notification_type.capitalize() }}</h1>
+      <p>
+        Sending {{ notification_type }} is an invitation&#8209;only feature.
+      </p>
+      <p>
+        If you want to try it out,
+        <a href="{{ url_for('.support') }}">get in touch with the GOV.UK Notify team</a>.
+      </p>
+
+      {% set
+        back_link_dict = {
+          'add_new_template': {
+            'url' : '.add_template_by_type', 'text': 'Back to add new template'
+          },
+          'templates': {
+            'url' : '.choose_template', 'text' : 'Back to templates'
+          },
+          'view_template': {
+            'url' :'.view_template', 'text' : 'Back to the template'
+          }
+        }
+      %}
+
+      {{ page_footer(
+        back_link=url_for(
+          back_link_dict[return_to]['url'],
+          service_id=current_service.id,
+          template_id=template_id),
+        back_link_text=back_link_dict[return_to]['text']
+        ) }}
+
+    </div>
+  </div>
+
+{% endblock %}