R1:7d849afd16c5

Add a "WorkboardCardTemplate" class to make workboard client code easier to reason about

Summary:
Depends on D20266. Boards currently have several `whateverMap<cardPHID => stuff>` properties, but we can just move these all down into a `CardTemplate`, similar to the recently introduced `HeaderTemplate`.

The `CardTemplate` holds all the global information for a card, and then `Card` is specific for a particular copy in a column. Today, each `CardTemplate` has one `Card`, but a `CardTemplate` may have more…