React with PR status on messages containing a GitHub PR link

[lensbart]

Is your feature request related to a problem? Please describe.
When someone posts a link to a GitHub PR, it would be nice to have a small indicator of the PR status. One approach would be to have PR status reaction images (e.g. )

Describe the solution you'd like
The GitHub Slack integration would react to messages containing a GitHub PR link. It would add a reaction for “draft”, “in review”, “approved”, “merged” and “closed”, and remove previous reactions that are not applicable anymore. For example, when a PR gets merged, the “approved” reaction gets removed and the “merged” reaction gets added.

Describe alternatives you've considered
We currently do this ad hoc (reacting with ✅ or ) to avoid that multiple people click the link to a PR that has already been approved, wasting invaluable deciseconds.