Progress reworking UsersController.

`UsersController` methods `#index`, `#new` and `#create` rewritten to
use second-gen DSOs and `ResultHandler` classes as appropriate.

One interesting buglet: note that in the spec for
`UsersController#create` where it checks mismatched
password/confirmation values, the identical error message is expected
twice. That matches ActiveRecord's handling of that. Explanations are
welcomed.

RSpec: 963 examples, 0 failures, 85 pending
RuboCop: 243 files inspected, no offenses detected