Currently the iOS version of SimplyE identifies libraries using numeric IDs from Accounts.json. We need to start using the UUIDs instead, because that's what the library registry uses. The UUIDs are also kept in Accounts.json, so there's no need to start talking to the library registry just to get this done.

This requires a migration for people who open a new version of SimplyE after using the old version. Even after the rest of Accounts.json is gone, we'll need to keep around a mapping of the UUIDs to the numeric IDs for quite a while.