Add default_sort_key field to users.

api/users.rb

@@ -48,7 +48,7 @@ put "#{APIPREFIX}/users/:user_id" do |user_id|
   if not user
     user = User.new(external_id: user_id)
   end
-  user.update_attributes(params.slice(*%w[username email]))
+  user.update_attributes(params.slice(*%w[username email default_sort_key]))
   if user.errors.any?
     error 400, user.errors.full_messages.to_json
   else

models/user.rb

@@ -6,6 +6,7 @@ class User
   field :external_id, type: String
   field :username, type: String
   field :email, type: String
+  field :default_sort_key, type: String, default: "date"
 
   has_many :comments, inverse_of: :author
   has_many :comment_threads, inverse_of: :author
@@ -62,7 +63,8 @@ class User
                         "follower_ids" => subscriptions_as_source.map(&:subscriber_id),
                         "id" => id,
                         "upvoted_ids" => upvoted_ids,
-                        "downvoted_ids" => downvoted_ids)
+                        "downvoted_ids" => downvoted_ids,
+                        "default_sort_key" => default_sort_key)
     end
     if params[:course_id]
       hash = hash.merge("threads_count" => comment_threads.where(course_id: params[:course_id]).count,