One-off error in end date detection

[messa]

https://github.com/messa/pyladies-courseware/blob/stable/frontend/pages/course.js#L74

When the end_date contains 2021-03-17, then on 2021-03-17 it already stops showing a button for entering the course.

[messa]

It is already being computed on backend: https://github.com/messa/pyladies-courseware/blob/b6429fd958cc9128c64bdf70a80191376418a8c3/backend/cw_backend/courses/courses.py#L152-L153

So would be better to use this flag in frontend instead of computing it again (and wrongly 😇) in frontend