[WIP] Spreadsheet validation

This is associated with moving inappropriate code out of config/settings.yml which is associated with the upload of orphan spreadsheets (https://osraav.atlassian.net/browse/OSRA-240)
I've created this pull request just to provide visibility of the approach I've been taking, to see if you want me to progress along these lines. 
Note that this will cause several rspec failures, because I haven't updated the tests to align with this module design. 
woops - I should be getting the class variable here instead of assigning @first_row to 4 directly.