Fix some of the most egregious errors in Harbormaster log paging

Summary:
Depends on D19141. Ref T13088. Some of the fundamental log behaviors like "loading the correct rows" are now a bit better behaved.

The UI is a little less garbage, too.

Test Plan: Viewed some logs and loaded more context by clicking the buttons.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19142