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