New `SessionNewAction` DSO, 1 of 2.

"The first 80% of the work takes 80% of the time; the last 20% of the
work takes the other 80% of the time", and "Fixes are often inversely
proportional to the error messages spewed by their lack" apply here.

SessionNewAction DSO now returns the Guest User rather than `nil` for
the result's `entity` field. Of course, to make that happen required a
bit more effort…

* `UserDao#all` now returns everything *except* the Guest User;
* `UserRepository#authenticate` has been extensively cleaned up;
* `UserRepository#all` IS NOT redundant with `UserDao#all` doing its
thing; apparently different ActiveRecord `#all` methods are involved;
* The UserDao table is now seeded with the Guest User;
* Spec coverage updated to match.

RSpec: 922 examples, 0 failures (but see Issue #87)
RuboCop: 222 files inspected, no offenses detected