Add JSON response format to entropy endpoints (#2673)

* Add JSON response format to entropy endpoints

Add account settings JSON endpoint returning account info including auto_postpone_period. Add auto_postpone_period to board JSON responses. Add JSON update support to both account and board entropy controllers.

* Add auto_postpone_period to all board response examples in API docs

* Use auto_postpone_period_in_days in JSON responses and API docs

* Use valid period values in permission tests

test/controllers/boards_controller_test.rb

@@ -258,6 +258,7 @@ class BoardsControllerTest < ActionDispatch::IntegrationTest
     get board_path(boards(:writebook)), as: :json
     assert_response :success
     assert_equal boards(:writebook).name, @response.parsed_body["name"]
+    assert_equal boards(:writebook).auto_postpone_period_in_days, @response.parsed_body["auto_postpone_period_in_days"]
   end
 
   test "show as JSON includes public_url when published" do