Add basic PagesController with Devise login requirement

I need to demonstate that Devise is working, so I need a page
that requires login, and another that doesn't.

Create a basic PagesController with two actions and associated routes.
A Railsy resource would do the trick but would be more complex.  We
really just need two actions.

Also:

* Install jQuery, because the vanilla app generated by "rails plugin
  new" doesn't seem to include that.

* Silence the i18n warning: [deprecated] I18n.enforce_available_locales
  will default to true in the future. If you really want to skip
  validation of your locale you can set I18n.enforce_available_locales =
  false to avoid this message.

* Restrict 118n version to ensure enforce_available_locales= is
  supported

* Restrict devise to ~> 3.3, which is when "Sign in" was changed to
  "Login in" in sessions/new.html.erb.  This is needed for tests to
  pass.  (https://github.com/plataformatec/devise/commit/67d8dd1e)

* Remove the placeholder capy_spec now that (a) it fails and (b) we have
  something more meaningful to test for