Enhance Slack notifications and automate GitHub assignment replies based on issue labels

[iamshobhraj]

Summary

Updated the existing notify_team_new_comment workflow.
This update adds the following steps :-

1. distinguish comments on "help wanted" and "non help wanted" labelled issues and based on the label, send notification to a slack channel.
2. In "Non help wanted" labelled issues, send a bot reply if the comment by external users contains a certain keyword while ensuring that there was no bot message sent in past hour on that issue.
3. "Help wanted" labelled issues work just like before.

I've tested this by creating a testing environment.

References

addresse issue #23

[MisRob]

Thanks a lot @iamshobhraj! I've just successfully tested learningequality/.github#29 and merged it. I've used changes in this PR for testing too, even though I won't merge it since I renamed some files. This repository is only for playing around so doesn't matter.

I will now close learningequality/.github#23 - wonderful work, much appreciation.