This prep commit extracts the logic for reformatting Slack mentions into
Zulip mentions from `convert_to_zulip_markdown` into a new helper
function, `convert_slack_workspace_mentions`.
This is done to make the reformatting logic be reusable else where such
as in the Slack webhook or Slack incoming webhook.
(cherry picked from commit 4b472611d0)
2d9c167ca3