Fix #N/A appearing on agency number widgets

- This was caused by Google Spreadsheets running the command to import data from the transition spreadsheet and not getting data. This made `#N/A` appear for unpredictable lengths of time, unpredictably, so this hopefully successfully deals with it by querying the spreadsheet every two seconds until the todo and done values are no longer `#N/A`, or another variant `#VALUE!`.
- Do some tidying up to make `SPREADSHEET_ROOT` a constant and include the worksheet key in it, while I'm here.