Make Harbormaster objects destructible

Summary:
Ref T13114. See PHI511. Ref T13072. This makes Buildables, Builds, Targets and Artifacts destructible with bin/remove destroy.

This might not be totally exhaustive. In particular:

• File artifacts won't destroy the file. This is sort of okay because file artifacts are currently just a file reference, but probably shouldn't be how things work in the long term.
• BuildCommand doesn't get cleaned up, but BuildMessage does on Build. See T13072 for more.

Test Plan: Used bin/remove destroy to nuke a bunch of builds, buildables, etc. Loaded stuff in the web UI and it all looked like it got nuked properly.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13114, T13072

Differential Revision: https://secure.phabricator.com/D19269