Permalinks should be thread-aware

[srid]

Permalinks currently point to the channel message listing. But if the referenced message is in a Slack thread it should refer to a listing that is dedicated to that thread only.

• Add thread:<threadid> support to search
• Make permalinks pointing to inside of a thread as /search/thread:<threadid>/<msgid>
• Optionally, provide a link to the channel list as "View this message in the context of its channel".