Route for adding new application member

- domain method for creating a new application member
- ApplicationInvitations domain class
- nested form for adding a new user that holds user data, application
  permission sets, and environment roles
- Invitation service can infer invitation type based on role it's given
- new invitation email templates

atst/forms/data.py

@@ -1,3 +1,4 @@
+from atst.models import CSPRole
 from atst.utils.localization import translate, translate_duration
 from atst.models.environment_role import CSPRole