Mattermost Bot - Post an issue link in a channel based on a label

[pavlenex]

I've been discussing with @britttttk about how we can improve processes for certain teams on mattermost.
For example, we've been thinking of forming a testers team, a group of people who can help us test PR's after they've passed code review.

Example: Maintainer reviews a PR, puts Ready for testing label. A link to that PR is posted in #Testers channel on Mattermost notifying people there's a new PR to test. I think this process would be helpful for other teams like "UX/UI" and "Design" for #UX/UI channel. The labels should work on organization lvl if possible (though this may be impossible) so "Ready for testing" can be put on both website, directory, main repo, docs, etc.