Ensuring to properly restore and instantiate Column Selector

- Column settings service updated to provide a slightly more elaborate column id but ensuring it is not unique so that it can be referenced and matched when restoring saved settings
- In settings controller watch of "spreadsheet.url" value, it now conditionally checks the oldValue to ensure it was not undefined or an empty string before resetting columns to []. It would be undefined or "" if this is the initializing phase of the settings app. This conditional is to avoid clobbering previously saved column settings when restoring saved settings