Add email-sending feature in usertask.py
Merge branch 'master' of https://github.com/Phoenix1708/OpenAcademy_OpenStack_Flyway
Refactor usertask.py, add db_handlers.users.py and add functions in helper.py

1) Separate all DB transactions from usertask.py to db_handlers.users.py
2) Methods about email are all migrated to helper.py