First whack at User model validation.

Password and password confirmation must match (courtesy of
`has_secure_password`); pen name must be present and non-empty (but has
no length restrictions yet).

RSpec: 135 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 48 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 4
User model now rejects null passwords on create action.

RSpec: 136 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 48 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 4
New Users controller, with accompanying :new action view.

* Controller/specs now have traditional #new and empty #create methods;
* Routing is updated to allow User :new and :create actions;
* New-User Slim template with form;
* Specs for new-user template (exhaustively) validates form layout;
* I18n updated with non-default submit button text;
* Debugged :be_a_form_label RSpec matcher. D'oh!

RSpec: 165 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 51 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 4
Users controller #create specced and implemented.

RSpec: 170 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 51 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 4