[Performance] Bulk apply of page flags - UX

It is observed that the bug apply of page flags takes significant time of more than 5 seconds before submission itself.

Also; there needs to be loading indicator and based on the response state. In case of error; the modal to be shown.

Start by locating the bulk page-flags action and its modal submission flow, then reproduce the more-than-five-second delay before submission and inspect the response states. Done means the operation responds faster where possible, shows a loading indicator during processing, and displays the modal when the response reports an error.

Written by the indexing model from the issue text.