(core) Adding latest version section to the admin panel.

Summary:
Update for the admin page to show the latest available version information.
- Latest version is read from docs.getgrist.com by default
- It sends basic information (installationId, deployment type, and version)
- Checks are done only on the page itself
- The actual request is routed through the API (to avoid CORS)

Test Plan: Added new test

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D4238

test/server/customUtil.ts

@@ -64,3 +64,31 @@ export async function serveSomething(setup: (app: express.Express) => void, port
   const url = `http://localhost:${port}`;
   return {url, shutdown};
 }
+
+/**
+ * Creates a promise like object that can be resolved from outside.
+ */
+export class Defer {
+  private _resolve!: () => void;
+  private _reject!: (err: any) => void;
+  private _promise: Promise<void>;
+
+  constructor() {
+    this._promise = new Promise<void>((resolve, reject) => {
+      this._resolve = resolve;
+      this._reject = reject;
+    });
+  }
+
+  public get then() {
+    return this._promise.then.bind(this._promise);
+  }
+
+  public resolve() {
+    this._resolve();
+  }
+
+  public reject(err: any) {
+    this._reject(err);
+  }
+}