In the future, an `application_invitation1 will not refer to a `user` until
someone accepts the invitation; they'll only reference an
`application_role`. When a user is invited to an application, the
inviter can specify the environments the invitee should have access to.
For this to be possible, an `environment_role` should reference an
`application_role`, because no `user` entity will be known at that time.
In addition to updating all the models and domain methods necessary for
this change, this commit deletes unused code and tests that were
dependent on `environment_roles` having a `user_id` foreign key.
- 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