Add sort/filter by need_date to Cage and Litter admin pages

[danieldkato]

It's not obvious how to do this as need_date is a custom method, not a database field.

One workaround would be to create an actual database field for need_date, then have that field updated as part of LitterAdmin's save_model method that gets called every time someone hits the "save" or "save and continue editing" button.

There may be some other ways using querysets that allow us to sort non-database field custom methods though.

[cxrodgers]

Let's deprioritize this for a little bit since it sounds complicated, and also it sounds like Drew/Kate will prefer to get this sort of information from a Census view.

[cxrodgers]

Now that the Cage admin page functions as a "Census view", it would be useful to have this ability to sort by "need_date" on the Cage admin page. Then we would have the "Things to do now" view that we need for this milestone.