Support for slack mrkdwn

It would be helpful if the Slack receiver permitted Slack mrkdwn in the text of Slack notifications. This would allow us to add mentions and text hyperlinks to the body of Slack notifications. This could significantly improve the readability of our slack alerts.

Prometheus makes this configurable using the [mrkdwn_in setting for the Slack receiver.](https://prometheus.io/docs/alerting/latest/configuration/#slack_config)

Start by locating the Slack receiver and the code that formats notification text. Review Prometheus's mrkdwn_in setting for the expected behavior, then verify that mentions and text hyperlinks are preserved in Slack notifications and covered by appropriate tests.

Written by the indexing model from the issue text.