Summary:
New language selector on the Account page for logged-in users.
New icon for switching language for an anonymous user.
For anonymous users, language is stored in a cookie grist_user_locale.
Language is stored in user settings for authenticated users and takes
precedence over what is stored in the cookie.
Test Plan: New tests
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D3766
