Support reactivating users

We're working on adding support for users to the Juju Terraform provider (see https://github.com/juju/terraform-provider-juju/pull/148) and as part of this, there was a question about whether we should actually deleted users, or if we should just disable them. The reason why this question came up is that if you remove users currently they're not actually removed, they're just permanently and irreversibly disabled. I believe this is for auditing purposes.

This could have pretty catastrophic outcomes if you mistakenly delete users managed via Terraform and then can no longer recreate them.

This was previously filed as https://bugs.launchpad.net/juju/+bug/1630728 with the subject "support re-activate users" but that was changed to "clarify removeuser message" and then marked fix released. Rather than reopen that bug I thought it best to create a new one.