Previously, if you tried to invite a user whose account had been
deactivated, we didn't provide a clear path forward for reactivating
the users, which was confusing.
We fix this by plumbing through to the frontend the information that
there is an existing user account with that email address in this
organization, but that it's deactivated. For administrators, we
provide a link for how to reactivate the user.
Fixes#8144.