Add bin/harbormaster write-log to write some arbitrary content into a new Harbormaster log

Summary: Ref T13088. This is currently minimal but the modify-execute development loop on build logs is extremely long without it.

Test Plan: Ran echo hi | ./bin/harbormaster write-log --target 12345, saw the log show up in the web UI.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19130