Add report on number of new user registration, by day

[ywarnier]

In main/admin/statistics/index.php, add a new report on the number of new users registrations, by day or month.

For some use cases, it is important to get a clear idea of the frequency and speed of registration of new users. Registration dates are stored in user.registration_date.

Extra: on the same page, provide statistics on the spread of new users by creator_id (with the name of the creator).

To make this report more useful, add a selector for a date range.

Show a chart (days vs number of users)

[marshel]

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.

[ywarnier]

In the context of Chamilo 2.0, this issue also includes reviewing all other reports in main/admin/statistics/index.php