Tag Tracking+: Reduce API load when multiple Tumblr tabs are open

### Concept It would require a fairly clever use of extension storage, at least in all of the iterations I've thought of so far, but we should be able to share data between Tumblr tabs to prevent Tag Tracking+ from generating additional unnecessary API load when multiple tabs are open. This is especially important if a user follows many, many tags. Opening multiple Tumblr tabs in succession can currently cause up to hundreds of API requests in quite rapid succession.