Add ability to delete placeholder users in UI
About
#519391 is adding support for deleting placeholder users through CSV.
This issue is to add support to delete placeholder users in the UI.
Proposal
Add new buttons that allow deleting. Next to placeholders that have the Not started reassignment status.
It would be like below but instead of Delete buttons they would be Trash icon buttons #537340 (comment 2478107215):
Click to see design - but use **Trash icon** instead of **Delete button**
When the button is clicked, open a modal confirmation box that the user clicks before the deletion happens.
The modal content is #537340 (comment 2477648051).
Technical proposal
Backend
The backend would add a new GraphQL mutation within the existing Mutations::Import::SourceUsers namespace.
The mutation would call the service to delete the placeholder User record and Import::SourceUser record that was added as part of #519391.