Increase number of sites allowed

Right now we are storing all site data as a JSON [user property](https://developers.google.com/apps-script/guides/properties) `SITES`. As we have started persisting more data about sites (specifically post types) we are starting to run into the limits for individual keys.

To allow for more sites, the persistence layer needs to be refactored to separate each site into its own property.

Start by locating the persistence layer and the user property named SITES, then review how site data and post types are stored. Refactor the storage design so each site uses its own property while preserving existing site data, and verify that more sites can be persisted without exceeding individual-key limits.

Written by the indexing model from the issue text.