The `image_comment_temp` table was created to temporarily hold the new fields needed for {T166732} so it wouldn't have to be held up waiting for an expensive schema change on Commons' `image` table. The plan was always to eventually merge it into the `image` table once we had time to do that schema change.
Thanks to T187089, that `image` schema change is happening sooner than we had thought.
The rough plan here is:
[X] Write (and merge) the patch to add the `img_descrption_id` field to the `image` table, and the maintenance code to migrate data from `image_comment_temp` to `image`, and the changes to CommentStore to
[X] Actually perform the schema change on WMF sites (already in progress, see T187089)
[X] Configuration changes to start writing `img_description_id`.
[] Run the maintenance.
[] Configuration changes to stop writing `image_comment_temp`.
[] Write (and merge) the patch to drop the `image_comment_temp` table and the code in CommentStore for using it.