Portfolio invitations do not associate a user entity until the
invitation has been accepted. User info, including DOD ID, is held on
the invitation itself. When a user accepts and invitation, their user
entry is associated with the corresponding `portfolio_role` entry.
The same change will be applied to `application_role` and application
invitations. For now, small changes have been made to
application-related methods so that that flow works as-is.
- Adds override to portfolio landing page access check to see if user
has access to any applications within the portfolio.
- Route for accepting an application invitation redirects directly to
portfolio applications route.
- Tests ensure application user only sees apps the user has access to on
the portfolio landing page.
