Prepopulate "decide on need" form with details of the old status

This PR changes the "decide on need" form to be pre-populated with the details of the existing need status. This change was triggered by feedback from @wryobservations and @fofr that not having the previous need status details was confusing for the user.

The implementation cleans up `NeedsController#update_status` and other places by extracting a `NeedStatus` object (/cc @vinayvinay).