(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

app/server/lib/DocSession.ts

@@ -122,7 +122,7 @@ export function getDocSessionUser(docSession: OptDocSession): FullUser|null {
     const user = getUser(docSession.req);
     const email = user.loginEmail;
     if (email) {
-      return {id: user.id, name: user.name, email, ref: user.ref};
+      return {id: user.id, name: user.name, email, ref: user.ref, locale: user.options?.locale};
     }
   }
   if (docSession.client) {