Support "scopes"

[zachahn]

ActiveAdmin has a feature called scopes. The UI shows each scope in a tab-like interface.

For example, one tab might show you all of the active records (no pun intended!), and the other might show you soft-deleted records. Or it might make sense to have one tab per region.

This scoping feature will be separate from the existing filtering feature; a scope is not a collection of filters, it's its own thing. But filters will be able to further filter down the current scope.

Is there a better name for this?