Accept JSON in Admin::BroadcastMessagesController

Alex Pennellsrequested to merge
broadcast-messages-json-controller into master

What does this MR do and why?

Related issue: #368847 (closed)

As the broadcast messages page is converted to a Vue app, we'll want to have JSON endpoints available for create and update. This MR adds support for JSON requests in addition to the existing HTML requests.

I also separated the feature pushing into a before action, as it will be needed for both the index and edit pages

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