R1:f114b2dd7d4b

When viewing a live build log, trap users in a small personal hell where nothing but slavish devotion to the log exists

Summary: Depends on D19152. Ref T13088. This adds live log tailing. It is probably not the final version of this feature because it prevents escape once you begin tailing a log.

Test Plan: Used `bin/harbormaster write-log --rate ...` to write a log slowly. Viewed it in the web UI. Clicked "Follow Log". Followed the log until the write finished, a lifetime later.

Subscribers: PHID-OPKG…