(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/Client.ts

@@ -84,7 +84,8 @@ export class Client {
   constructor(
     private _comm: any,
     private _methods: any,
-    private _host: string
+    private _host: string,
+    private _locale?: string,
   ) {
     this.clientId = generateClientId();
   }
@@ -102,6 +103,10 @@ export class Client {
     return this._host;
   }
 
+  public get locale(): string|undefined {
+    return this._locale;
+  }
+
   public setConnection(websocket: any, reqHost: string, browserSettings: BrowserSettings) {
     this._websocket = websocket;
     // Set this._loginState, used by CognitoClient to construct login/logout URLs.