Invalidate console session when minio user doesn't exists

[Alevsk]

Testing

• Create new user and assign policy:

./mc admin user add orcadmin alevsk alevsk12356
./mc admin policy set orcadmin mcsAdmin user=alevsk

• Login to console and list buckets/users/policies etc
• Delete the user using mc

./mc admin user remove orcadmin alevsk

• Try to navigate some more in Console, you should be redirected to the login screen since your session will be invalidated

Fixes #306

[dvaldivia]

Will this log out the user on the UI?

[cesnietor]

testing.

[Alevsk]

Will this log out the user on the UI?

yes, in the UI user will be redirected to the login screen, we already have some logic to do that

[cesnietor]

Tested, just if the user is disabled it won't redirect to login page but as discussed previously this will be investigated.
LGTM.