Because:
* The 500 page was a server template outside React and Fluent
* The 502, 503 and update_firefox server pages were not needed
* The CI extract job cannot parse ?? in content-server server files
This commit:
* Adds a Fluent ServerError page that fxa-settings shows on a 500
* Serves that page from the content-server 500 handler for HTML
* Fixes the JSON and text 500 branches to use req.gettext
* Removes the 500, 502, 503 and update_firefox server routes
* Uses ES2018 syntax in the beta-settings spec
Closes FXA-6656