Use EditEngine, not CustomFields, to define Dashboard Panel edit behavior

Summary:
Depends on D20371. Ref T13272. Dashboard panels use CustomField to specify editable panel behavior. This is an older approach which was largely or entirely obsoleted by EditEngine.

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.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20372