Add welcome method to UserMailer

[Refs #72499504]

Added #welcome method, created welcome mail template.
Update RegistrationsController spec

[Refs #72499504]

Added test for #create action. Test checks if welcome mail for user was
sent.
Update registraions_controller_spec

[Refs #72499504]

Refactored spec a bit.
Update routes.rb

[Refs #72499504]

Put resources :users after devise_for.