Resolve "在 PM 的 slack channel 中发送 milestone 剩余 issue 通知"

What does this MR do and why?

• 添加新的定时任务 remind-milestone-ending-in-slack-for-pm
• 添加新方法 pm_team_members
• 添加新方法 jihu_remind_milestone_ending_in_slack_for_pm 和 pm_members_to_remind_milestone_ending
• 修改发送到 test channel 的测试消息

Closes #140 (closed)