[JUJU-732] Fix external user login errors on single controller

[wallyworld]

When trying to login as an external user, the controller was attempting to find this user in state and returning a not found error. This is wrong because external users do not exist in the model, only their permissions.

Also, some structs and interfaces associated with entity auth were poorly named, eg AgentAuthenticator is actually also used for users.
And the AuthInfo struct has an obsolete LastConnection field that was unused.

QA steps

juju bootstrap --config identity-url=https://api.jujucharms.com/identity --config identity-public-key=hmHaPgCC1UfuhYHUSX5+aihSAZesqpVdjRv0mgfIwjo=
juju grant wallyworld@external login
juju grant wallyworld@external write default

export JUJU_DATA=/path
cd $JUJU_DATA
cp ~/.local/share/juju/controllers.yaml $JUJU_DATA
juju login
juju deploy ubuntu
juju status

Also check local users using juju add-user, juju grant, and juju register.

[ycliuhw]

lgtm ty

[wallyworld]

$$merge$$