There are 244789 PageState entities in the datastore consuming 211MB. The PageState model contains only a `value` BlobProperty. There is currently nothing that deletes old PageStates.

Let's add a timestamp to the PageState model so we can track when it was last accessed so we can delete unused entities.
We don't need to know exactly when it was last accessed, just the date or week, and we should try not to slow down /short_uri too much, so we don't need to update the timestamp every time it's accessed, just if the timestamp is older than a day or week.