Add user delete and bulk user delete actions to user list

[bcomnes]

Blocked by https://github.com/netlify/gocommerce-js/issues/20

After we add the capability to delete a user and users in bulk, we need to add a ui element to gocommerce admin to take advantage of this feature.

Mockups: https://sketch.cloud/s/pQJwk/PzLDz8

https://github.com/netlify/gocommerce-js/issues/20 https://github.com/netlify/gocommerce/issues/136 https://github.com/netlify/gocommerce/pull/149