Convert frontend code to typescript (#31559)

None of the frontend js/ts files was touched besides these two commands
(edit: no longer true, I touched one file in
61105d0618
because of a deprecation that was not showing before the rename).

`tsc` currently reports 778 errors, so I have disabled it in CI as
planned.

Everything appears to work fine.

web_src/js/features/repo-migrate.ts

@@ -0,0 +1,63 @@
+import {hideElem, showElem} from '../utils/dom.ts';
+import {GET, POST} from '../modules/fetch.ts';
+
+const {appSubUrl} = window.config;
+
+export function initRepoMigrationStatusChecker() {
+  const repoMigrating = document.querySelector('#repo_migrating');
+  if (!repoMigrating) return;
+
+  document.querySelector('#repo_migrating_retry').addEventListener('click', doMigrationRetry);
+
+  const task = repoMigrating.getAttribute('data-migrating-task-id');
+
+  // returns true if the refresh still needs to be called after a while
+  const refresh = async () => {
+    const res = await GET(`${appSubUrl}/user/task/${task}`);
+    if (res.status !== 200) return true; // continue to refresh if network error occurs
+
+    const data = await res.json();
+
+    // for all status
+    if (data.message) {
+      document.querySelector('#repo_migrating_progress_message').textContent = data.message;
+    }
+
+    // TaskStatusFinished
+    if (data.status === 4) {
+      window.location.reload();
+      return false;
+    }
+
+    // TaskStatusFailed
+    if (data.status === 3) {
+      hideElem('#repo_migrating_progress');
+      hideElem('#repo_migrating');
+      showElem('#repo_migrating_retry');
+      showElem('#repo_migrating_failed');
+      showElem('#repo_migrating_failed_image');
+      document.querySelector('#repo_migrating_failed_error').textContent = data.message;
+      return false;
+    }
+
+    return true; // continue to refresh
+  };
+
+  const syncTaskStatus = async () => {
+    let doNextRefresh = true;
+    try {
+      doNextRefresh = await refresh();
+    } finally {
+      if (doNextRefresh) {
+        setTimeout(syncTaskStatus, 2000);
+      }
+    }
+  };
+
+  syncTaskStatus(); // no await
+}
+
+async function doMigrationRetry(e) {
+  await POST(e.target.getAttribute('data-migrating-task-retry-url'));
+  window.location.reload();
+}