Allow Harbormaster build plans to be tagged with projects and searched by tag

Summary: Ref T10457. This is mostly just for consitency, but I imagine it will make managing large/complex build processes easier, and if we support Herald rules it would eventually let you write "Build plan's tags include [whatever]" to apply behavior to a group of plans.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10457

Differential Revision: https://secure.phabricator.com/D15360