feat: allow disabling default redirect, set status to 503

[KaelWD]

Changes

• Default status code is now 503 not 404. There is some discussion about it in Traefik always returns a 404 HTTP code even if no backend is currently configured traefik/traefik#8141 but I think 404 is a really bad default, 503 will actually display a "this server is down" page behind cloudflare for example.
• Default config can now be removed so you can override it with something custom such as a pretty HTML page.
• Refactored the setupDefaultRedirect method to only do things once, it was a bit hard to follow before.

Issues

• fix Default Redirect 404 File (Dynamic) #3774

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• Should I add a proxy->redirect_rewrite option too to display a custom page without changing the url?

[KaelWD]

Is there a better way to make sure this is true by default?

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