[frontend] set webhook_rebuild even in APIv3
    
    The `process_package_add_or_edit` function is used for both
    APIv3 and APIv1. The APIv1 accesses some data through
    `flask.request.form`, however APIv3 uses JSON everywhere and takes
    user input from elsewhere. I am updating the function to be able to
    set `webhook_rebuild` property even when used through APIv3.
    (While preserwing the backwards compatibility)
    
    At first it looked like the `process_package_add_or_edit` function
    is going to be same for both APIs, so we could save some code by
    using the existing function. However, there are more and more changes
    and we should have this re-implemented for APIv3.