broadcast: add ability for admin to lock enable button

If there's a circumstance where admin needs to perform some debugging/maintenance and we want to prevent the customer from re-enabling a broadcast, it'd be useful if we had an enable lock feature. This would stop users from switching this and would have a small text field that says "Enable locked due to maintenance".

Start by locating the broadcast enable control and the admin settings or state handling it uses. Confirm how an administrator can lock enabling, how customers are prevented from re-enabling the broadcast, and where the maintenance message should appear; done means the locked state and text are visible and effective.

Written by the indexing model from the issue text.