Resolve "Remind "Retrospective feedback" in Slack"

What does this MR do and why?

As !52 (merged), in this merge request, I add two modules and some config files to realize the relative issue. Schedule pipeline will execute jihu_remind_retrospective_feedback_in_slack method every Friday, to remind "Retrospective feedback" in slack.

How to set up and validate locally

Use bundle exec rspec to run unit test to validate locally.

Closes #36 (closed)