(core) User language switcher

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

package.json

@@ -144,7 +144,6 @@
     "https-proxy-agent": "5.0.1",
     "i18n-iso-countries": "6.1.0",
     "i18next": "21.9.1",
-    "i18next-fs-backend": "1.1.5",
     "i18next-http-middleware": "3.2.1",
     "image-size": "0.6.3",
     "jquery": "3.5.0",