Verify email addresses of new users

[paulmueller]

It would be nice to verify the email address of a new user, i.e. send an email with a verification token or somesuch.

This has been proposed in several places in the past already:

• Verify new users by email ckan#1535
• Improvements to User Management ckan#2838
• https://lists-archive.okfn.org/pipermail/ckan-dev/2019-November/023331.html

[paulmueller]

It will be much simpler to write a CKAN extension that facilitates this feature once IUserForm ckan/ckan#6070 is implemented. The task of the extension would be to check if there is a new user (or new email address) and then generate and handle a verification link somehow.