Associating a person with an existing user

[ildar170975]

Assume there are person entities - jane, john, ...
And there is only one user - "John Smith" associated with person.john.
How can we add a new user "Jane Smith" associated with person.jane?

Currently a person entity may be associated with a NEW user only:

1. Open a window to configure a person entity.
   Toggle "Allow person to login":
   

2. A window to create a new user is opened:
   
   where it is not possible to select an existing user.

There is a possible way - but in yaml only:

1. Find out a user_id of some user:
   

2. Add this ID to an existing person entity:

person:
  - name: Jane
    id: jane
    user_id: b2e8d127662949ef924a34239302ae42

[steverep]

Duplicate of #9811