feat(gc): record event type for manifest tasks queued due to an upload

Related to #487 (closed). Please read the issue for context.

This is the first step towards identifying the triggering event for online GC review tasks. Here we add the new event column to the gc_manifest_review_queue table and update the first of four triggers responsible for inserting rows in this table - gc_track_manifest_uploads so that it starts filling this column with the corresponding event identifier - manifest_upload.