Update Backup/restore settings to/from a file

https://github.com/mozilla/multi-account-containers/pull/1533#discussion_r910173590

src/js/options.js

@@ -66,7 +66,9 @@ async function backupContainers() {
     const content = JSON.stringify(
       await browser.runtime.sendMessage({
         method: "backupIdentitiesState"
-      })
+      }),
+      null,
+      2,
     );
     backupLink.href = `data:application/json;base64,${btoa(content)}`;
     backupLink.download = `containers-backup-${(new Date()).toISOString()}.json`;