FeatureSpecLoginHelper updated; UserEntity cleaned

UserCreatorData now internal to LoginHelper (as that's the only class
that would use it now), and LoginHelper takes a hash of attribute
values.

UserEntity specs now brought up to RSpec 3, using standard
`described_class` rather than home-baked `klass`.

RSpec: 710 examples, 0 failures
RuboCop: 178 files inspected, no offenses detected
Feature specs exercising user name validation.

RSpec: 714 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected