Resolve "Remove email confirmation notification on onboarding pages"

Closes #1022

Both registration and trial registration process should be covered. The notification message is from set_confirm_warning of a concern ConfirmEmailWarning. We can use show_confirm_warning? to mute this message for the controllers we want to.

This email unconfirmed message will be only added to HTML GET actions, we can override this on controller level, no need to choose actions.

Proposal

1. override in registrations/welcome_controller.rb
2. override in registrations/projects_controller.rb

On Boarding Pages

1. /users/sign_up/welcome
2. /users/sign_up/projects/new?namespace_id=84&trial=false
3. /users/sign_up/welcome/continuous_onboarding_getting_started?project_id=24
4. /users/sign_up/welcome/trial_getting_started?learn_gitlab_project_id=24

welcome	new project	continuous_onboarding_getting_started	trial_getting_started