Wired Action::NewSession into SessionsController.

The `#new` method is probably simpler to read (though I suspect the
McCabe score will be higher); it sets up a fire-and-forget action.

RSpec: 911 examples, 0 failures, 131 pending
RuboCop: 232 files inspected, no offenses detected
Action::CreateSession wired into controller.

* New `ActionCreateSession` code and specs;
* `SessionsController` beginning to show visible change;
* `SessionsController` spec *function* unchanged; database
class/factory changes only

When the specs tell you that the code you thought you copied, pasted
and fiddled with doesn't pass the same specs that the earlier code did,
believe the specs and find your mistakes. :grimace:

RSpec: 921 examples, 0 failures, 121 pending
RuboCop: 234 files inspected, no offenses detected