Show banner mentioning migration (#960)

On Oct 26th II will undergo subnet migration. The banner gives users a
heads up.

src/frontend/src/banner.ts

@@ -4,6 +4,15 @@ import { anyFeatures } from "./features";
 // Show a warning banner if the build is not "official". This happens if either the build
 // is a flavored build, or if the origin is not 'identity.ic0.app'.
 export const showWarningIfNecessary = (): void => {
+  showWarning(html`Internet Identity is moving to a new subnet and will be
+    partially unavailable on October 26th!
+    <a
+      class="features-warning-btn"
+      target="_blank"
+      rel="noopener noreferrer"
+      href="https://forum.dfinity.org/t/internet-identity-subnet-migration-on-october-26/15989"
+      >more</a
+    >`);
   const officialUrl = "https://identity.ic0.app";
   if (anyFeatures()) {
     showWarning(html`Test only. Do not use your regular anchor!