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/__init__.py

@@ -83,6 +83,10 @@ from app.notify_client.template_statistics_api_client import (
     template_statistics_client,
 )
 from app.notify_client.user_api_client import user_api_client
+from app.url_converters import (
+    LetterFileExtensionConverter,
+    TemplateTypeConverter,
+)
 from app.utils import format_thousands, get_logo_cdn_domain, id_safe
 
 login_manager = LoginManager()
@@ -229,6 +233,8 @@ def init_app(application):
         }
 
     application.url_map.converters['uuid'].to_python = lambda self, value: value
+    application.url_map.converters['template_type'] = TemplateTypeConverter
+    application.url_map.converters['letter_file_extension'] = LetterFileExtensionConverter
 
 
 def convert_to_boolean(value):