Make "source up-to-dateness" work with dates in the future

[fniessink]

Use cases:

• Actions that need to be done before a certain date
• Limited validity of documents

Source up-to-dateness should report the number of days left when the date is in the future.

Source up-to-dateness currently gets the date from the source and calculates the number of days since the reported date: max(0, (now() - reported_date()).days). If we remove the max we get a negative number for reported days in the future. Negative days are probably too confusing for users. Investigate whether we can make it work by using the "direction" to distinguish between the two uses of the metric.