Allow the author of an initiative to choose the end of signature collection period

As an admin, I want to be able to enable/disable the possibility for users to choose their own end of signature collection period when creating their initiative. As an author of an initiative, I want to be able to select an end of signature collection period when editing the step 4 of my initiative creation form. After the initiative has been published, the author can't change the end of signature collection period. After the initiative has been published, the admin can change the end of signature collection period.