bench: broadcast page shouldn't resubmit form on reload.

When a form is submitted, it's generally good practice to either go to another URL, or clear the form and display a success message.
We should implement one of these options on the broadcast page.

I propose we initially have the broadcast config displayed as a view only UI with labels, then when a user clicks "edit" it loads admin/broadcast/edit and turns the config labels into config form inputs and they can be edited. A user can then either click "save" which submits the new config and loads admin/broadcast, or cancel, which takes them back to admin/broadcast without saving.

Start by tracing the admin/broadcast page and the admin/broadcast/edit flow described in the issue, focusing on what happens after form submission and browser reload. Implement either a redirect to admin/broadcast or a cleared form with a success message; the proposed view-only, edit, save, and cancel behavior should work without resubmitting on reload.

Written by the indexing model from the issue text.