Extract logic from NeedsController#update_status

... into a NeedStatus class.
Wrap Need API status response in an object

This allows encapsulating logic in the NeedStatus class.
Extract common reasons for invalidity to NeedStatus
Reuse NeedStatus constants in status change form
Refactor form field name for clarity
Display previous status when changing need status
Update need status form JS to work with initial values
Merge pull request #136 from alphagov/prepopulate-new-need-status-form-with-old-status

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