Create pipeline to import source users from user_contribution.ndjson for Offline Transfer

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Create an NDJSON pipeline to create or update Import::SourceUser data from user_contributions.ndjson in offline exports. This pipeline should only be executed in offline transfer, not direct transfer. Currently, user_contributions.ndjson is exported but not used in direct transfer. This new pipeline does not need to block the execution of other pipelines as existing pipelines will create Import::SourceUser records if one doesn't exist for the contribution

Note: There may eventually be some refactors to direct transfer to make use of user_contributions.ndjson to prevent importing contributions from blocked or banned users. As of 18.5, there's been no additional changes. See #508111 for more details.