Remove background.js?

At this point, it looks like `background.js` is not being used for anything. I believe it's safe to remove in the current incarnation of the extension.

Alternatively, we could keep `background.js` and use it as a data provider for clips. That would involve updating `sidebar.js` to send messages to `background.js` for this purpose instead of reading/saving clips itself.

Let me know which approach you'd like me to take.