Improve live Harbormaster log follow behaviors

Summary:
Depends on D19166. Ref T13088. When the user scrolls away from a followed log, break the focus lock.

Let users stop following a live log.

Show when lines are added more clearly.

Don't refresh quite as quickly give users a better shot at clicking the stop button.

These behaviors can probably be refined but are at least more plausible and less actively user-hostile than the first version of this behavior was.

Test Plan: Used write-log --rate to write a large log slowly. Clicked "Follow Log", followed for a bit. Scrolled away, still got live updates but no more scroll lock. Clicked stop, no more updates.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19167