[Development] Add Day-Streak tracking

[BartoszBrodowski]

Add a feature that let's the app track day-streak. Count the streak when user does at least one little thing like reading one subject/finishes an exercise.

It will be connected to the callendar on the main page.

A nice thing would be to make changes in all days that the user was active, not only when the person is on a streak. Days on streak would be indicated with a different color of the dot.

[kacperkruger]

I have decided to divide this topic into smaller parts to make it easier to review:

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