Consolidate local storage keys on setup

[maxxcrawford]

          Ignorant Q: But do these have to be set separately, or can we set one object w/ multiple keys?

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/storage/StorageArea/set implies we can almost condense these four set() calls into a single call w/ object w/ 4 keys. 🤷

Originally posted by @pdehaan in #530 (comment)