Summary:
Depends on D20406. Ref T13272. This gets about half of Dashboards working with the new "duplicate panel friendly" storage format. Followups will fix the write pathways.

Collateral damage here includes:

  - Remove the old Dashboard/Panel edge type. We have a new, more general edge type for "container X uses panel Y", and we don't need this edge type for anything else.
  - Remove "attachPanels()" from Dashboard. Only rendering actually needs this, and it can just load the panels.
  - Remove "attachPanelPHIDs()" from Dashboard. We can look at the panel refs to figure this out.
  - Remove "attachProjects()" from Dashboard. Nothing uses this and it's not a very modern approach.
  - `getPanelPHIDs()` just looks at the config now.
  - Deleted some `LayoutConfig`-related code which is broken/obsolete.

Test Plan:
  - Viewed various dashboards which were created before the changes, saw them render correctly.
  - Viewed a dashboard with two of the same panel! AMAZING!

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20407