Allow Harbormaster build plans to be disabled

Summary: Fixes T4187. Ref T1049. Allows build plans to be enabled or disabled.

Test Plan: Enabled and disabled build plans.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4187, T1049

Differential Revision: https://secure.phabricator.com/D7822