Resolve "Password set emails for Users created by API"

Open username-removed-812482 requested to merge duk3luk3/gitlab-ce:21848-user-service into master

What does this MR do?

  • Extracts user creation logic from admin area controller to a service
  • Adds API functionality
  • Obsoletes !6637 (closed)

Why was this MR needed?

  • Common feature request (#21848 (moved))
  • Adds equivalent API for Webapp action

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #21848 (moved)