(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/server/lib/Authorizer.ts

@@ -373,6 +373,14 @@ export async function addRequestUser(dbManager: HomeDBManager, permitStore: IPer
     mreq.users = [dbManager.makeFullUser(anon)];
   }
 
+  if (mreq.userId) {
+    if (mreq.user?.options?.locale) {
+      mreq.language = mreq.user.options.locale;
+      // This is a synchronous call (as it was configured with initImmediate: false).
+      mreq.i18n.changeLanguage(mreq.language).catch(() => {});
+    }
+  }
+
   const meta = {
     customHostSession,
     method: mreq.method,

app/server/lib/DocSession.ts

@@ -122,7 +122,7 @@ export function getDocSessionUser(docSession: OptDocSession): FullUser|null {
     const user = getUser(docSession.req);
     const email = user.loginEmail;
     if (email) {
-      return {id: user.id, name: user.name, email, ref: user.ref};
+      return {id: user.id, name: user.name, email, ref: user.ref, locale: user.options?.locale};
     }
   }
   if (docSession.client) {

app/server/lib/FlexServer.ts

@@ -59,7 +59,7 @@ import {startTestingHooks} from 'app/server/lib/TestingHooks';
 import {getTestLoginSystem} from 'app/server/lib/TestLogin';
 import {addUploadRoute} from 'app/server/lib/uploads';
 import {buildWidgetRepository, IWidgetRepository} from 'app/server/lib/WidgetRepository';
-import {readLoadedLngs, readLoadedNamespaces, setupLocale} from 'app/server/localization';
+import {setupLocale} from 'app/server/localization';
 import axios from 'axios';
 import * as bodyParser from 'body-parser';
 import express from 'express';
@@ -1278,10 +1278,7 @@ export class FlexServer implements GristServer {
   }
 
   public getGristConfig(): GristLoadConfig {
-    return makeGristConfig(this.getDefaultHomeUrl(), {
-      supportedLngs: readLoadedLngs(this.i18Instance),
-      namespaces: readLoadedNamespaces(this.i18Instance),
-    }, this._defaultBaseDomain);
+    return makeGristConfig(this.getDefaultHomeUrl(), {}, this._defaultBaseDomain);
   }
 
   /**

app/server/lib/sendAppPage.ts

@@ -65,6 +65,7 @@ export function makeGristConfig(homeUrl: string|null, extra: Partial<GristLoadCo
     namespaces: readLoadedNamespaces(req?.i18n),
     featureComments: process.env.COMMENTS === "true",
     supportEmail: SUPPORT_EMAIL,
+    userLocale: (req as RequestWithLogin | undefined)?.user?.options?.locale,
     ...extra,
   };
 }
@@ -111,9 +112,12 @@ export function makeSendAppPage(opts: {
     const customHeadHtmlSnippet = server?.create.getExtraHeadHtml?.() ?? "";
     const warning = testLogin ? "<div class=\"dev_warning\">Authentication is not enforced</div>" : "";
     // Preload all languages that will be used or are requested by client.
-    const preloads = req.languages.map((lng) =>
-      readLoadedNamespaces(req.i18n).map((ns) =>
-        `<link rel="preload" href="locales/${lng}.${ns}.json" as="fetch" type="application/json" crossorigin>`
+    const preloads = req.languages
+      .filter(lng => (readLoadedLngs(req.i18n)).includes(lng))
+      .map(lng => lng.replace('-', '_'))
+      .map((lng) =>
+        readLoadedNamespaces(req.i18n).map((ns) =>
+       `<link rel="preload" href="locales/${lng}.${ns}.json" as="fetch" type="application/json" crossorigin>`
       ).join("\n")
     ).join('\n');
     const content = fileContent