V3: better user creation/deletion and test boilerplate

• unify user creation on the api.common.users package, with create_user and delete_user functions.
  • removes copied code between api.bp.register and manage.cmd.users.
• clean test boilerplate. all setup and teardown of the test users is now done via pytest fixtures. this completly removes the need of hardcoded test users (the hi and admin users aren't needed as we now just create a random user, then set it as admin, or not, etc).
• add user-auto-delete when we failed to send the register email right after.