Rename the sticky nav overflow menu to something that doesn't conflict with menus named 'More'

[benlk]

Changes

• The HTML id for the overflow menu in the sticky nav is now #sticky-nav-overflow

Why

From #1336:

If the name of the Global Navigation menu slugifies to "more", then the slug of the menu name causes a conflict somehow. Changing the name of the identifier for the overflow menu when we create it in navigation.js fixes the problem, and because it's contained all within one file, can be deployed without worrying about cache conflicts between Javascript and PHP.