Store and display sleep needed for past nights #619

It would be nice to be able to view the calculated sleep needed for past nights.
Currently the "Sleep" ring on the today page only shows the "blue ring" for the most recent night. In my opinion it would be nice if the amount that is needed would be stored in the database for every day so the rings are displayed correctly for the past nights.
Then it would also be possible tp show stats like "Sleep Sufficiency" in the health tab.