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

templates/emails/portfolio/invitation.txt

@@ -0,0 +1,10 @@
+{% extends "emails/base.txt" %}
+
+{% block content %}
+
+Join this JEDI Cloud Portfolio
+{{ owner }} has invited you to join a JEDI Cloud Portfolio.  Login now to view or use your JEDI Cloud resources.
+
+{{ url_for("portfolios.accept_invitation", token=token, _external=True) }}
+
+{% endblock %}