Allow adding broadcast templates

At the moment the page is the same as for text message templates,
except:
- different H1
- no guidance about personalisation, links, etc (until we decide how
  these should work)

For now you won’t be able to really create a broadcast template, because
the API doesn’t support it (the API will respond with a 400). But that’s
OK because no real services have the broadcast permission yet.

This required a bit of refactoring of how we check which template types
a service can use, because there were some hard-coded assumptions about
emails and text messages.

app/utils.py

@@ -461,10 +461,6 @@ def get_time_left(created_at, service_data_retention_days=7):
     )
 
 
-def email_or_sms_not_enabled(template_type, permissions):
-    return (template_type in ['email', 'sms']) and (template_type not in permissions)
-
-
 def get_logo_cdn_domain():
     parsed_uri = urlparse(current_app.config['ADMIN_BASE_URL'])