Create GraphQL mutation to revoke source user mappings

Background

Part of #478147. No API currently lists source users by reassign_to_user; SourceUsersFinder is namespace-scoped and requires :admin_namespace.

New scope on Import::SourceUser filtering by reassign_to_user and completed status.
New currentUser field (e.g. importSourceUsers) returning the viewer's completed mappings, paginated. New policy ability (e.g. read_own_import_source_user in Import::SourceUserPolicy) granted when viewer == reassign_to_user.
New mutation Import::SourceUsers::Revoke, consistent with existing mutations in app/graphql/mutations/import/source_users/. Authorizes a new revoke_import_source_user ability (viewer == reassign_to_user, status completed).
Behind the feature flag from #629219 (closed).