Modify project structure and rename some functions

1. Modify description in pipeline-schedule.yml.
2. Rename function next_versioned_milestone_title in jihu_milestone_helper.rb.
3. Modify plugins structure, add jihu_slack.rb.
4. Move file related to slack from /lib to /lib/slack.
5. Move .yml file related to slack from policies/stages/hygiene to policies/stages/hygiene/slack