Move main menu to dropdown

Motivation

On small mobile screens, the main menu takes a a large portion of the screen leaving only a small area for content. On an iPhone SE 3rd gen, for example, the usable section of the screen is less than half - screenshot.

Changes

This MR moves the main menu items into a dropdown menu which is collapsed by default.

• before
• after
• menu-opened

Related Issues

• #272
• lego#14

Remaining Work

This MR does not fully address the issue described in lego#14 , it is still a little strange on those pages to have a main menu, breadcrumbs, and a sub-menu.