(core) Simple localization support and currency selector.

Summary:
- Grist document has a associated "locale" setting that affects how currency is formatted.
- Currency selector for number format.

Test Plan: not done

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D2977

app/common/BrowserSettings.ts

@@ -4,4 +4,6 @@
 export interface BrowserSettings {
   // The browser's timezone, must be one of `momet.tz.names()`.
   timezone?: string;
+  // The browser's locale, should be read from Accept-Language header.
+  locale?: string;
 }