Further improvements of group imports history API
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This is issue for furthre improvements of group imports history API
* ATM, backend for retrieving bulk imports history provides items in chronological order - first item is ~~oldest~~ latest import. ~~This is counter-intuitive to user expectations, so we need either flip an order (simplest solution) or add ~backend parameter for controlling order~~

* Filtering by source groups is missing
* Author name is missing from API
* ~~Original import host is missing from API response, so we're unable to render link to original group on remote instance~~ - `source_type` is available in [migration details endpoint](https://docs.gitlab.com/ee/api/bulk_imports.html#get-group-or-project-migration-details).
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