Optimize current purchase window updating

[AndrewLester]

Currently updateCurrentPurchaseWindow fetches all purchase windows in order to loop through them and check which one is the current window. It does this through a call to getDocuments with the collection name 'purchase_windows'.

This call could be replaced by a call to getDocumentsWhere with a condition to only find purchase windows that have the same dayOfWeek as the current day. Specify this condition in the function's queryOnQueryable parameter. Something like:

(queryable) => queryable.where('dayOfWeek', '==', new Date().getDay());