Dropdowns in the main navigation for nested menu items

Problem/Motivation

When the main navigation menu includes nested menu items, the menu should use a dropdown to make it possible to navigate to these items.

Steps to reproduce

After applying the Haven site template, apply the Events Calendar recipe
Your site will now have content at /events and /event/calendar, so add menu items for these
Make the Calendar menu item a child (nested under) the Events menu link
Go to the home page and observe that it is not possible to navigate to the Calendar
Manually update the browser path to /events/calendar and observe that the menu is identical to how it looks when at the Events page

Proposed resolution

The theme should use dropdowns (with a disclosure button for accessibility) when the main navigation has nested menu links

Remaining tasks

User interface changes

API changes

Data model changes