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.

* Remove the placeholder capy_spec now that (a) it fails and (b) we have
  something more meaningful to test for
Install dotenv gem to store sensitive keys

Sensitive keys (like Janrain credentials) can be committed to public
version control.  And 12factor.net/config suggest extracting all
environment-specific configs to environment variables.

Setup dotenv to load them from the .env file.
Add janrain_uuid to User model

Most applications will integrate with Janrain by recording a user's
Janrain UUID.

Record ours in the users table.
wip: start installing devise capturable