Make Re-posts threads

When re-posting the questions, make them come in as a thread. It will be tricky.

https://api.slack.com/incoming-webhooks

https://api.slack.com/docs/message-threading

Start by reading the Slack Incoming Webhooks and message-threading documentation linked in the issue, then locate the code that re-posts Stack Overflow questions. Done means re-posted questions arrive in Slack as threads rather than standalone messages.

索引モデルが issue の本文から書いたものです。