We were using the internal UID of the channel in the hint, which would then fail to match any 'channel' configs in Discourse (which use the human-friendly `#channel` format). This was causing automatic threading for transcripts to fail.
When notifications about a topic are posted to multiple slack channels, and the Discourse channels are configured to "thread" the notifications, each channel will have a different thread_id. Previously we were only storing a single slack thread id per Discourse topic. This commit fixes that logic, so that threads in different channels are tracked separately.
