Add `"referenced"` and `"unchanged"` fields to statistics on Import Participant

[Borreg0]

Describe the issue

When importing participants from csv file, statistics for total, created, updated, new, error, structure levels and warning are calculated by backend. According to the new Participant Import Rework (OpenSlides/openslides-client#5124), a new kind of user "Referenced" should be added to be displayed along with the rest of statistics (On footer, imported summary dialog and as a field on filters).

This should be added to the backend in order to keep both functionality and logic together.

Payload from client (handle_request)

[{"action":"participant.json_upload","data":[{"data":[{"first_name":"a","structure_level":"Test structure level a","groups":"Delegates","number":"12345-67891","vote_weight":"1","gender":"female","username":"a","default_password":"a","is_active":"1","is_physical_person":"1","comment":"Test comment a"},{"first_name":"b","structure_level":"Test structure level b","groups":"Delegates","number":"12345-67892","vote_weight":"1","gender":"diverse","username":"b","default_password":"jKwSLGCk","is_active":"1","is_physical_person":"1","comment":"Test comment b as an external user"},{"last_name":"Administrator","structure_level":"Test structure level","groups":"Admin","number":"12345-67890","vote_weight":"1","gender":"male","username":"admin","default_password":"admin","is_active":"1","is_physical_person":"1","is_present":"1","comment":"Test comment"}],"meeting_id":4}]}]

Response from backend

"statistics": [
{"name": "total", "value": 3}, 
{"name": "created", "value": 0}, 
{"name": "updated", "value": 3},
{"name": "error", "value": 0},
{"name": "warning", "value": 0},
{"name": "structure levels created", "value": 3}, 
{"name": "groups created", "value": 0}],
 "state": "done"}
]]

Expected behavior
Receive a response like this:

"statistics": [
{"name": "total", "value": 3}, 
{"name": "created", "value": 0}, 
{"name": "updated", "value": 1},
**{"name": "referenced", "value": 2},**
{"name": "error", "value": 0},
{"name": "warning", "value": 0},
**{"name": "unchanged", "value": 3},**
{"name": "structure levels created", "value": 3}, 
{"name": "groups created", "value": 0}],
 "state": "done"}
]]

Additional context

I am currently building a workaround on client, but this cannot be a permanent solution

[luisa-beerboom]

I'm going to add some kind of "changed" flag into the fields, I'm not going to add it to the statistics or the row header data level.

[luisa-beerboom]

For list fields I'm using the changed flag for all added entries and have a field-to-amount-of-removed-entries dict list_deletions in the preview row that counts all removed entries.