Skipping of email validation as new option of registration workflow

Today users have a 5-step registration workflow. In step 3 the user email is entered and then validated via an email link (Double-Opt-In). The workflow continues only when clicking the email validation link. 

In some setups email verification is not needed. Email might be entirely optional or just should not be validated for whatever reason. 

Todo: 

• A new configuration is added to the self-registration configuration: validate email or skip email validation during self-registration
• The registration workflow is modified to the following: 
  • if email is validated
    • show email form in step 3
    • validate email
    • in step 4 show email as read-only field
  • if email is not validated
    • skip current step 3
    • in current step 4 show email as as input field
      • if email mandatory: make field mandatory
      • if email optional: don't make it mandatory
    • in this scenario the wizard has one step less than before