API4: write handleExpiredDocument

[cellisco]

code: opendataplus/dataset_modules/city-of-boulder-open-development-review-cases/handleExpiredDocument.js

After the dataset has been updated, the main platform will find any documents that haven't been updated in the dataset's retention time period. for each of these documents that haven't been updated (i.e., they were probably deleted from the source dataset), it will call the handleExpiredDocument module.

The handleExpiredDocument module should then graciously delete the data while respecting what the subscribed applications want to have happen to their data when the source public data no longer exists.