Move PHUIFormBoxView to PHUIObjectBoxView
Summary: I'd like to reuse this for other content areas, renaming for now. This might be weird to keep setForm, but I can fix that later if we need.
Test Plan: reload a few forms in maniphest, projects, differential
Reviewers: epriestley, btrahan
Reviewed By: epriestley
CC: Korvin, aran
Differential Revision: https://secure.phabricator.com/D7120