Adds menubar ARIA semantics to editor menu

[tespin]

Related to #2933, makes progress on @lindapaiste's #2968 (thanks for starting some of this already!)

Changes:

• moved role="menubar" from nav div to file menu
• moved role="menuitem" from <li> to<button> or <a>, whichever element performs a function
• added role="menuitem" to login and sign up links
• added aria-expanded to dropdowns
• updated snapshots
• updated index.integration.test.jsx to look for role='menubar' instead of role='navigation'

This PR tries to add ARIA menubar semantics to the editor navigation menu, to further support accessibility features like the ones described in issue #2933. The main challenge was resolving how to maintain both role=menubar and role=navigation on the page. ARIA guidelines differentiate on what makes a menubar (a widget offering a list of actions or functions) vs navigation (a collection of links used for navigating the web page or page content). Menubars are also more closely associated with the menus of desktop applications like Photoshop rather than typical examples of site navigation. Since the web editor's file menu functions more like an application than site navigation, I followed the W3C's menubar pattern where possible.

The biggest change is probably substituting the header <nav> element for a <div>; I gave role=navigation to the right portion of the nav bar since that dealt more closely with links to other pages. It may also make sense to wrap those lists in a <nav> element but I have to look into that more.

Since the menubar pattern implies specific keyboard behaviors, it would help to implement those features as a next step.

References:

• https://www.w3.org/WAI/ARIA/apg/patterns/menubar/examples/menubar-editor/
• https://www.radix-ui.com/primitives/docs/components/menubar
• https://adrianroselli.com/2017/10/dont-use-aria-menu-roles-for-site-nav.html

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

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[raclim]

Thanks so much for your work on this, this is really awesome!

I really appreciate how much thought and consideration went into this and the detailed insight into your process! I think a lot of these choices make sense to me, and would be a major improvement to what's there so far.

Since this seems good to me, I'm going to merge this in—please feel free to add any other amendments you'd like to make in a new PR!