Display serverids consistently as 8-char pubkey, or 6-char tubid.

Display serverids consistently as 8-char pubkey, or 6-char tubid.

This makes it easy to distinguish between old V1-Introducer
nodes (identified by their Foolscap TubID) and new V2 nodes (identified
by their ed25519 pubkey).

This fixes a few places where we used to display a tubid even if we had
a pubkey, making it hard to visually correlate servers in two different
displays. It also cleans up the way we pass serverids to the JS-based
download timeline.

The "introweb" subscribed-clients list still shows tubids.