Extract Spreadsheet model from app/utils.py

`app/utils.py` is a bit of a dumping ground for things we don’t have a
better place for.

We now have a place and structure for storing ‘model’ code (‘model’ in
the model, view, controller (MVC) sense of the word).

This commit moves the spreadsheet model to that place.

app/main/validators.py

@@ -11,7 +11,8 @@ from notifications_utils.template import BroadcastMessageTemplate
 from wtforms import ValidationError
 
 from app.main._commonly_used_passwords import commonly_used_passwords
-from app.utils import Spreadsheet, is_gov_user
+from app.models.spreadsheet import Spreadsheet
+from app.utils import is_gov_user
 
 
 class CommonlyUsedPassword: