(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/client/ui/UserManager.ts

@@ -622,7 +622,8 @@ function getFullUser(member: IEditableMember): FullUser {
     id: member.id,
     name: member.name,
     email: member.email,
-    picture: member.picture
+    picture: member.picture,
+    locale: member.locale
   };
 }