Deeplink to incident Slack channel from the incidents page

# Description

It'd be useful to have a deeplink (`slack://`) to the relevant conversation channel from the incidents page.

This would allow users to jump off from the incidents log without having to scroll back through Slack.

This requires the `TEAM_ID` as well as the conversation ID. 

## References

* [Slack: Deep Linking](https://api.slack.com/reference/deep-linking)