Return soft deadline from api

Author: nygrenh

app/controllers/api/v8/courses/exercises_controller.rb

@@ -70,6 +70,7 @@ def index
               publish_time: ex.publish_time,
               solution_visible_after: ex.solution_visible_after,
               deadline: ex.deadline_for(current_user),
+              soft_deadline: ex.soft_deadline_for(current_user),
               disabled: ex.disabled?,
               unlocked: exercises.include?(ex.id)
             }

app/models/exercise.rb

@@ -17,6 +17,7 @@ class Exercise < ActiveRecord::Base
     property :publish_time, type: :string, format: 'date-time', example: '2016-10-24T14:06:36.730+03:00'
     property :solution_visible_after, type: :string, format: 'date-time', example: '2016-10-24T14:06:36.730+03:00'
     property :deadline, type: :string, format: 'date-time', example: '2016-10-24T14:06:36.730+03:00'
+    property :soft_deadline, type: :string, format: 'date-time', example: '2016-10-24T14:06:36.730+03:00'
     property :disabled, type: :boolean, example: false
     property :available_points, type: :array do
       items do