[C2] Add report for course/session subscriptions by day

[ywarnier]

I would suggest also a report on course/session subscriptions by day.

As I've seen is only available by filtering by "user_subscribed" and 'session_add_user' on field track_e_default."default_event_type" and course id directly on mysql.

Would be nice if the same chart shows the users unsubscription as well, as many users may drop or be excluded.

Originally posted by @marshel in #3002

(Nice to have)

[christianbeeznest]

It is added with this commit dabec80

Thanks for confirmation.