Notify our slack on contributors' issue comments

[vkWeb]

Summary

This adds a github action that will notify our slack #dev-community channel when any contributor comments on a issue.

ToDo --

• Activate slack's webhook, already requested, need to be approved by one of our slack admins.
• Add SLACK_WEBHOOK_URL as a GitHub secret.
• Merge this PR and then test the action.

References

Closes #9521.

Reviewer guidance

I don't know how best to test this 🤷‍♂️

---

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0b6.dev0_git.579.g4969eaca.pex
Windows Installer (EXE)	kolibri-0.16.0b6.dev0+git.579.g4969eaca-unsigned.exe
Debian Package	kolibri_0.16.0b6.dev0+git.579.g4969eaca-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b6.dev0+git.579.g4969eaca-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b6.dev0+git.579.g4969eaca-0.1.0-debug.apk
TAR file	kolibri-0.16.0b6.dev0+git.579.g4969eaca.tar.gz
WHL file	kolibri-0.16.0b6.dev0+git.579.g4969eaca-py2.py3-none-any.whl

[rtibbles]

Possible tweaks here:

• Put the issue name instead of just 'issue'
• Make a link out of the issue name
• Update the link to include the reference to the specific comment so that clicking on it will open the issue at that specific comment
• Could also make the github actor text link to the user profile of the github user.

[vkWeb]

I've included the issue title and linked it to the specific comment. I have not added github profile user link because that would be unnecessary.

[vkWeb]

@rtibbles marked this ready to merge. 🎉

Here's the link to the slack app that I added -- https://api.slack.com/apps/A067L07B9QW/incoming-webhooks. We need to add a webhook to that app, some instructions here on that -- https://api.slack.com/messaging/webhooks and we also need to add that webhook as a github secret.

[MisRob]

Great, let's give it a try then? @rtibbles do you think you could help with setting Slack webhooks, please? We've tried earlier with @marcellamaki but neither of us has permissions.

[rtibbles]

Was there a reason not to use the offiicial Slack github action here? https://github.com/slackapi/slack-github-action

[rtibbles]

Readthedocs build seems to have stalled, merging anyway.

[vkWeb]

Was there a reason not to use the offiicial Slack github action here? https://github.com/slackapi/slack-github-action

No reason as such I guess, but I agree it would be better to use their official action, will fix it.