Backfill public links from the last few minutes when added to channel

If someone posts a b.corp link and then the bot is added to the channel within, say, 5 minutes, it would be nice to backfill that message by posting the public versions.

No files, tests, or entry points are identified in the issue. Start by locating the Slack channel-add handling and the code that converts b.corp links into public links, then determine how recent channel messages can be retrieved. Done means links posted within roughly five minutes before the bot was added are converted and posted publicly.

Written by the indexing model from the issue text.