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/main/views/sms.py

@@ -22,7 +22,11 @@ from app.main import main
 from app.main.forms import CsvUploadForm
 from app.main.uploader import s3upload
 
-from ._templates import sms_templates
+from ._templates import templates
+
+sms_templates = [
+    template for template in templates if template['type'] == 'sms'
+]
 
 
 @main.route("/services/<int:service_id>/sms/send", methods=['GET', 'POST'])