Add URL converters for template and file types

Sometimes we manually check that a URL parameter is in a required set.
Sometimes we don’t bother.

This commit adds a URL converter to do this so that:
- we don’t have to re-write the same code every time
- it’s easier to apply this check to other endpoints

This means endpoints that previously allowed a `template_type` or
`message_type` of `None` now 404. So I’ve had to add new routes for
with URLs that don’t include such parameters.

So this…:
```
/services/128b91b6-2996-4107-bb65-51b7c24a728d/notifications/sms.csv
/services/128b91b6-2996-4107-bb65-51b7c24a728d/notifications/None.csv
```

…becomes:
```
/services/128b91b6-2996-4107-bb65-51b7c24a728d/notifications/sms.csv
/services/128b91b6-2996-4107-bb65-51b7c24a728d/notifications.csv
```

This matches what we do for the HTML-responding equivalent (see
265931d217/app/main/views/jobs.py (L215-L216))

app/url_converters.py

@@ -0,0 +1,11 @@
+from werkzeug.routing import BaseConverter
+
+
+class TemplateTypeConverter(BaseConverter):
+
+    regex = '(?:email|sms|letter)'
+
+
+class LetterFileExtensionConverter(BaseConverter):
+
+    regex = '(?:pdf|png)'