Update usage logging: Track total usage time over a week, SP:4

Create a module that adds tracking for the total time a user spent using the app over a week.

Do the same as Update usage logging: Track average time per da... (#1035 - closed) with the following differences:

In #1035 (closed) we tracked the daily average usage time; in this issue we want the total time. This makes a difference if there are days without any usage at all.

Value to post: total number of minutes of usage during the last seven days.

Adjust label and descriptions accordingly.