Keep subscription date and add a danger zone from a date option

[Drumor]

Hello,

Here are two improvements we detect: 1)It should be great to save the subscription date of a student to a course. 2) It should be possible to remove the data from the student at a certain date. (e.g: remove the student before September 14). If it's an option of the danger zone, it's great.

Thx

[anthonygego]

See #165 too.