Added new User table per first step of 111.

RSpec: 812 examples, 0 failures (but see Issue #87)
RuboCop: 152 files inspected, no offenses detected
First whack at User DAO.

Bearing a remarkable similarity to the existing UserData ActiveRecord
model (minus most business-aware logic beyond barest validation), this
is really straightforward. Unlike the earlier class, this one *does
not* have a `registered` scope depending on an ID greater than one; the
concept of a Guest User is exclusively at a higher level now.

RSpec: 822 examples, 0 failures (but see Issue #87)
RuboCop: 154 files inspected, no offenses detected