Streamlined Hypothesis login for Via

When viewing a page through Via¹, we may want to provide a cookie-based way to short-circuit the OAuth-based login flow and log people in who have active sessions without requiring any extra action on their part. This may require some investigation to determine how feasible this is, and what benefit it would provide.

¹ This also apparently applies to “the bookmarklet [and] other related use cases” – I’m not quite certain what that means.