Add a "(prototype)" link to the standalone build log on build pages

Summary: Depends on D19149. Ref T13088. Since the new log requires a bunch of log reprocessing, the cutover is going to require at least some time for installs to run migrations. Add a link in the UI to ease the transition, smooth over some behaviors a little, and fix a fetch issue where we'd request past the end of the log (since this is now enforced).

Test Plan: Viewed a traditional Harbormaster build, saw links to the new standalone log pages.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19150