update discontinuedDate policy to support stale lists

[collinbarrett]

Edit: I also propose that lists whose such values are more than 730 days (2 non-leap years) ago could've had those dates auto-added to the "Discontinued" values as well, but that'd be merely a secondary bonus. - #439 (comment)

My goal for "discontinuedDate" was only to capture a date if the maintainer explicitly stated in some fashion that the project was being discontinued. If the project seems to have been abandoned, but was never stated as such, to rely on the SnapshotService updated dates. but, I can't send SnapshotService back in time...

[collinbarrett]

Updated wiki for discontinuedDate to say that the date can either be the date that the maintainer declared the project dead or the date that the project was last updated if that date is more than two years ago.

Eventualy, if FilterLists' SnapshotService runs for more than two years, it can auto-determine this. But, we'll cross that bridge if/when we get there. For now, we'll have to add discontinuedDates manually for stale projects.