## Because
- Only the content-server renders the /update_firefox page.
- FxA moves the post-verify pages to React in fxa-settings, and this page is part of that set.
## This pull request
- Adds an UpdateFirefox page in fxa-settings, with Fluent strings, a story, and a unit spec.
- Serves /update_firefox from React behind the postVerifyOtherRoutes flag. get-update-firefox.js stays as the server fallback.
- Keeps /download_firefox on the server. The button links to it with the allowed query params and the flow data.
- Logs flow.begin and flow.update-firefox.view on mount.
- Keeps a raw `+` in forwarded query values. The button links straight to https://www.mozilla.org/firefox/download/thanks/ when context, deviceId, or flow data is missing, because /download_firefox returns 400 without them.
## Issue that this pull request solves
Closes: FXA-6657