GC INCOMPLETE users

[yarikoptic]

According to admin interface we have 95 INCOMPLETE users with dates going back to 2022

IMHO we should prune such records after e.g. a week. IMHO we should

• announce in the user registration form that accounts which did not complete registration would be "forgot" after e.g. 1 week
  • ref: Update new user registration questions #2038
• send out an email whenever we GC such an INCOMPLETE User account

NB Functionality to remove a user would be needed/desired if some user would decide to "be forgotten" and reaches out to us. So thinking about such functionality might be helpful in that case as well