Wisperised UsersController#create action.

* Parameter validation broken out into (a group of) classes.
* User entities are now sortable; this was added as a debugging aid
while tracking down another series of problems.
* Action error message reporting reworked to make grouping easier.
* Beginning to wonder about Rubocop file counting again.

RSpec: 992 examples, 0 failures, 107 pending
RuboCop: 246 files inspected, no offenses detected