Generate UserMailer

[Refs #72499504]

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.

Update mailer config

[Refs #72499504]

Added smtp settings, added gmail_name/password to secrets.yml

Update controller_spec, routes

[Refs #72499862]
Changed syntax.