Remove `lang` from fxa-client and partial requests

This removes lang from places where we were previously relying on the browser's `navigator.language` instead of accept headers.

@shane-tomlinson r?
Ha, looks like it only worked because the Markdown parser automatically creates an `id` with that name based on the header text, and that probably would have failed in any other locale than en-US.
For example, if you go to https://accounts.stage.mozaws.net/fr/legal/terms then the French version of "#terms-of-service" would be:

```html
<h4 id="conditions-d-utilisation">Conditions d’utilisation</h4>
```