Let dashboard panel types use customfield to manage editing

Summary: Ref T3583. Use the same approach Harbormaster does to give panels cheap forms.

Test Plan:
{F149218}

{F149219}

{F149220}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3583

Differential Revision: https://secure.phabricator.com/D8919