Add Practice sessions section into student stats

[effa]

It should show activity of the student in time, ideally with selectable time-window (last week, last month, last year, since the beginning). It can be e.g. a graph, where on the x-axis there is time (days/weeks/months/years) and on the y-axis there is time spent by training, visually grouped into sessions (rectangle per session).

There should somewhere be information about number of solved tasks as well. One possibility is to use y-axis for both number of tasks and for the time (and use bars for sessions and an interpolated line for time). I think this is the way it is/was on the KA, so we can inspire there.

For small time window, it can even visually distinguish individual tasks within a session (and show different color for solved/unsolved).

There can also be horizontal "goal" line to motivate for let say 2 practice sessions per day.