Add optional signup.

app/assets/stylesheets/sections/login.scss

app/controllers/registrations_controller.rb

app/models/user.rb

app/views/devise/passwords/new.html.erb

app/views/devise/registrations/new.html.erb

app/views/devise/registrations/new.html.haml

app/views/devise/sessions/new.html.haml

app/views/notify/new_user_email.html.haml

config/gitlab.yml.example

config/initializers/1_settings.rb

config/routes.rb

spec/mailers/notify_spec.rb

spec/requests/admin/admin_users_spec.rb

spec/requests/api/users_spec.rb