We've had the UploadReference table for some time now in core,
but it was added after ChatUpload was and chat was just never
moved over to this new system.
This commit changes all chat code dealing with uploads to create/
update/delete/query UploadReference records instead of ChatUpload
records for consistency. At a later date we will drop the ChatUpload
table, but for now keeping it for data backup.
The migration + post migration are the same, we need both in case
any chat uploads are added/removed during deploy.
Follow up to 766bcbc684
Makes ChatMessage.last_editor_id and ChatMessageRevision.user_id
NOT NULL since they are always filled in now and the last commit
had a migration to backfill this data.
Martin Brennan
Roman Rizzi