Enable users to find the currently-selected object in the collection content tree

[HaSistrunk]

Is your feature request related to a problem? Please describe.

If users select an item, and then continue scrolling, it is easy to lose where that selected item was in the tree.

Describe the solution you'd like

1. Implement darker focus/active styles for selected items.
2. Implement locate within collection button that scrolls back to the selected item on the page. Consider accessibility here. If the button only appears when the user scrolls away, is it necessary to announce to a SR that it has appeared? Maybe not, because a SR will not have the same disconnect between what is selected, and what is visually available on the screen.

Design solution: https://projects.invisionapp.com/share/3Z10HL9DXPBK#/screens