Automatically update term when upload calendar

[heyuhang0]

This should fix the occasional term issue described in #12 #15 by automatically updating the term value in firebase when someone uploads a new calendar.

Although the MR also makes it possible for one corrupted upload to break the whole site:

1. this will not affect existing subscription
2. it can be easily fixed by uploading another calendar
3. the whole system is already based on the trust in users

The MR also added a HELP button on the upload page which points to https://github.com/huixiang01/guide_to_sutd_timetable/blob/master/README.md