Convert Fieldmanager_DraggablePost to Fieldmanager_Multipicker

[netaustin]

Should use Fieldmanager_Datasource, which could be tags or users. Perhaps also allow the usage of 'limit' instead of assigning it directly, but make it limit the number of things you can drag in.

[mboynes]

@netaustin is this still valid?

[netaustin]

Yep—the draggable post class and UI is still there, and it's actually a very cool alternative to autocomplete or select, but it does need to be refactored to use datasource, or perhaps factored out into its own module. I favor the latter for expedience and simplicity.

@xmattus is this still in production?

[mboynes]

I started a new version of this called Fieldmanager Zones. I might replace DraggablePost with that but haven't decided yet.