Import rework Preview

[MSoeb]

Description:
Given the new possibilities with participant-import:
It is worth discussing if the preview should only the data from the file, or whether it should show preexisting data.

Somethings which needs discussion:

• how to handle data from the file which is new
• how to handle data from the file which is old
• how to handle the preview if the file is empty (at this place) but the data field is filled in the backend
• how to handle the preview if the file is filled (at this place) but the data field is ALSO filled in the backend

What shoud be discussed/happen:

• The preview should include all existing user data

[Elblinator]

The second wanted behaviour (The preview should include all existing user data) is not implementable without a new concept.
So far this was actively not the desired behaviour, because the preview should only show data which was from the imported file.
After some internal discussions that part will become it's own Issue or clarifying actions will be updated here.

[luisa-beerboom]

I extracted point 1 into its own issue #2457
I am changing the labels to this issue.

[luisa-beerboom]

I also took out the project labels as I understood them to be applicable to the bug part of what was initially asked.
If that is wrong @Elblinator @MSoeb please add them back

[Elblinator]

I'll move my comment to the bug Issue and deelte it in here