Add a prototype email template

If the templates page contains text messages and emails then there’s two ways it
could be structured:
- into two sections, all text messages first, then all emails
- emails and text messages interleaved, sorted by date

I think the second one is better. Imagine a situation where you mostly do emails
but have a few text messages. You’d have to scroll past the text messages to get
to your emails. Every time.

I reckon that the most commonly accessed templates will be the most recent ones.

app/templates/views/edit-template.html

@@ -15,8 +15,8 @@ GOV.UK Notify | Edit template
 			{{ textbox(form.template_body, highlight_tags=True) }}
 			{{ page_footer(
 				'Save and continue',
-				back_link=url_for('.dashboard', service_id=service_id),
-				back_link_text='Back to manage templates'
+				back_link=url_for('.manage_templates', service_id=service_id),
+				back_link_text='Back to templates'
 			) }}
 		</form>