Support simple display name setting on user registration via query param

[gabek]

There's been a lot of confusion in the past about how you can manually set somebody's chat display name at registration time. While I think how it currently works works is fine, it's clear we need an additional method.

I'm proposing something super-simple, just a query param. https://owncast.example.com/?displayname=gabe. And this would do one of a couple things:

1. If the requesting user has never previously been registered, it attempts to register with that name.
2. If the requesting user has already been registered, the server ignores this new re-registration request and does nothing.

So the second time somebody uses that query param on an Owncast instance, nothing happens.
Simple and obvious.

[costajohnt]

I've opened #5042 for this. Mind assigning the issue to me for the checklist bot? Happy to adjust the param handling if you had something different in mind.