Use EditEngine, not CustomFields, to define Dashboard Panel edit behavior
Throw away all the CustomField edit stuff. Convert the "text" panel to EditEngine to prove this at least mostly works.
This breaks "query" panels and "tab" panels (they'll still work fine, but they can't be meaningfully edited). I'll restore those in a future change.
Test Plan: Created and edited a "text" panel.
Reviewed By: amckinley
Maniphest Tasks: T13272
Differential Revision: https://secure.phabricator.com/D20372