In the data picker, make the interaction for schema browsing more like the new interaction for browsing collections

[mazameli]

Conceptually, browsing through a DB's schemas to see the tables inside is really similar to browsing through collections and subcollections to see the saved questions inside.

But with the new saved question picker, we're now using two different idioms for these interactions: accordion menus and sub-panels for schema browsing; and an expandable tree structure with side-by-side panels for collections.

Since we just implemented the new saved question browser, we should presumably make schema browsing more like that (rather than vice versa).

[mazameli]

Closing as resolved by the new EntityPicker component.