(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/server/lib/DocSession.ts

@@ -24,6 +24,7 @@ export interface OptDocSession {
 
 export function makeOptDocSession(client: Client|null, browserSettings?: BrowserSettings): OptDocSession {
   if (client && !browserSettings) { browserSettings = client.browserSettings; }
+  if (client && browserSettings && !browserSettings.locale) { browserSettings.locale = client.locale; }
   return {client, browserSettings};
 }