Add option to show 3rd level links on hover

Problem/Motivation

Having to expand the 3rd level by clicking makes it harder to find what you're looking for.

Proposed resolution

Show 3rd level in a flyout on desktop when vertical.

When the 2nd level buttons are clickable, use the toggle button to expand on hover. Otherwise, only click to expand on mobile.

Remaining tasks

Patch and review.

User interface changes

Option to expand 3rd level on hover.

Data model changes

Config schema change.