Add license headers across the project

ts/util/callingPermissions.ts

@@ -1,3 +1,6 @@
+// Copyright 2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
 export async function requestCameraPermissions(): Promise<boolean> {
   if (!(await window.getMediaCameraPermissions())) {
     await window.showCallingPermissionsPopup(true);