allow user account creation on profile edit forms

currently, if a profile includes the option to create a user account, those fields will only display on profile create forms – not on profile edit forms.

I have found there is great benefit in enabling it on edit forms. specifically, you can send an email to existing contacts that do not yet have a user account and invite them to create an account – using a profile edit link with checksum. the form will prepopulate with their contact information and you are ensured the user account will link to the correct contact record (no reliance on dedupe rules).

the patch for this is pretty trivial. I'd like someone from the core team to consider/approve the improvement and then I'll file a PR.