Handle 201 CREATED response to POST /users.json

[kueda]

As a part of the upcoming required email validation, creating a new user account will not be the same thing as authenticating that user, because after the account is created, the user cannot authenticate until they have verified their email address. Because of this, a successful POST /users.json request will respond with a 201 status to indicate the user was created, and a JSON body like

{
  "message": "A message with a confirmation link has been sent to your email address. Please follow the link to activate your account."
}

If POST /users.json includes a locale key in the request body, the message in the response should be localized.

So here are some todos:

• [x] The app should include the user's / device's locale when making the request to create the account
• [x] The app should respond to a 201 response status by showing the user a message and not signing them in
• [x] The message should be the message in the JSON response

[alexshepard]

done in dc1a9a9ef4f9b869f6f1ea83c05f318e2cd1472d