Lb/update languages menu ux behind flag

[lilybarrett]

Scope

We want to redesign the header with a two-tier menu that, among other things, reimagines what the Languages button/menu looks like. This adds that redesign behind the feature flag that also enables Smartling translations .

Asana Ticket:
https://app.asana.com/1/15492006741476/project/1213994587940653/task/1216081563934501?focus=true

Implementation

• Please note that this adds some important changes to the Typescript code that handles interactions w/ the headers/menus on Dotcom -- namely to allow Dotcom to handle multiple tiers of headers rather than just one.
• Additional CSS classes and design updates behind the feature flag -- You may notice some code duplication here. This isn't super pretty, but will ideally make it easier to remove old code once we enable the flag on production!

Screenshots

How to test

• Pull down this branch or navigate to the deployment branch
• Navigate to /_flags and turn on the feature flag
• Interact with the new two-tiered header -- ensuring that buttons/links on both menu tiers still work as expected, changing the screen size, etc.
• Turn on Voiceover and tab around, ensuring that the Languages button adheres to accessibility expectations. (It should behave similarly to the existing buttons on the menu).

Note: Changes to the footer will come in future work, once designs are complete.