Provide API read access to Harbormaster build logs

Summary:
Depends on D19150. Ref T13088. Allow clients to retrieve information about build logs, including log data, over the API.

(To fetch log data, take the filePHID to file.search, then issue a normal GET against the URI. Use a Content-Range header to get part of the log.)

Test Plan: Ran harbormaster.log.search, got sensible-looking results.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19151