First feature spec re-enabled.

Pinging Issue #118; we now use ActiveAttr to add ActiveModel-compatible
bits to `UserEntity`. Note that this also changes the semantics of what
gets passed back rom the `users/new` form; an obscurely-documented
(IMO) option on `form_for` saves the day.

I want my day over again.

RSpec: 1071 examples, 0 failures, 45 pending
RuboCop: 247 files inspected, no offenses detected