Persist notification dismissal server-side with Users::Callout

What does this MR do and why?

Adds server-side persistence to the dismissal of the notification banner. As we have observed in some issues, cookies for our app can be evicted seemingly at random, and users may frequently change browsers, OS versions or devices, causing the dismissal cookie to be lost. I believe there is a small-but-vocal set of users that would be driven insane by not being able to get rid of a notification, so I think if we can add server-side persistence to the dismissal per-user, this would dramatically improve the experience for these users.

This uses the built-in dismissal_path parameter for the broadcast message component to submit a request to the Users::Callout model instead. It's a bit of a hack, but have confirmed it works on my local machine.

Related to !154944 (merged)

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