(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/models/UserManagerModel.ts

@@ -63,6 +63,7 @@ export interface IEditableMember {
   name: string;
   email: string;
   picture?: string|null;
+  locale?: string|null;
   access: Observable<roles.Role|null>;
   parentAccess: roles.BasicRole|null;
   inheritedAccess: Computed<roles.BasicRole|null>;