(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/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