Allow logging in with devise

[grantspeelman]

Make Authenticating a User https://github.com/shakacode/cypress-on-rails/blob/master/docs/authentication.md part of the gem.

Using Rails engines we can make the following work out of the box.

  // after logging in we redirect the user
  cy.forceLogin({email: 'someuser@mail.com', redirect_to: '/home'})

  // If a different model is used instead of `User`
  cy.forceLogin({email: 'someuser@mail.com', model: 'Account'})

  // logging in by id
  cy.forceLogin({id: 1})

• If the login fails we should display a readable error message
• If devise is not being used we should display a readable error message