Issue964limitSizeOfStoredBackgroundTaskHistory

Added a custom save method to the BackgroundTask table to prevent the number of rows exceeding the value in the max_background_task_rows field in the new BackgroundTaskMaximumRows singleton table. Need to create a simple page for an administrator to modify this value. Refs issue #964

Added a form, view and entry in the Config menu for an OpenREM administrator to be able to change the BackgroundTaskMaximumRows singleton table. Refs issue #964

Adding form to update size of stored task history. Updated task documentation. Refs issue #964