Resolve "Remind "Capacity planning" in Slack"

What does this MR do and why?

In this merge request, I add two modules and some config files to realize the relative issue. Schedule pipeline will execute jihu_remind_capacity_planning_in_slack method every Friday, to remind "Capacity Planning" in slack.

Screenshots or screen recordings

How to set up and validate locally

Use bundle exec rspec to validate that all function can run well.
You can also use local GDK and test channel in slack to validate. This needs to create an APP in slack workspace.

MR acceptance checklist

• Add function jihu_send_slack_message to send message to slack channel in JihuSlackHelper.
• Add function slack_mention_user_str and slack_mention_user_group_str in JihuSlackHelper to get a string which fit the syntax.
• Add rspec test.
• Validate locally.

Closes #35 (closed)