Refactor Identity Verification methods

What does this MR do and why?

This MR cleans up some code related to Identity Verification (the following points are separate commits):

• Clean up ee/spec/features/registrations/identity_verification_spec.rb:
  • move saas tag from top describe block to where it is needed: the Trial flow describe block
  • Remove SubscriptionPortalHelpers module include and associated methods from Subscription flow examples
  • Remove stub_feature_flags(soft_email_confirmation: false) as this is not needed anymore
  • Reword the shared examples from the user with xxx risk to a xxx risk user with
• Rename check_user_confirmation to complete_identity_verification from EE::SessionsController
• Rename sign_in_and_redirect_or_confirm_identity to sign_in_and_redirect_or_verify_identity from OmniauthCallbacksController
• Move identity_verification_enabled? method from Users::EmailVerification::SendCustomConfirmationInstructionsService to User

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.