[Mixcloud] Harmonize ID generation from lists with full ID generation (#27787)

[Mixcloud] Harmonize ID generation from lists with full ID generation (#27787)

Mixcloud IDs are generated as `username_slug` when the full ID dict has been
downloaded.  When downloading a list (e.g. uploads, favorites, ...), the temporary
ID is just the `slug`.  This made e.g. archive file usage require the download
of stream metadata before the download can be rejected as already downloaded.

This commit attempts to get the uploader username during the GraphQL query, so the
temporary IDs are generated similarly.