Convert dashboard read/display pathways to the new panel storage format

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