allow /favicon.ico to be a valid route

[Rich-Harris]

closes #4968. It's impossible to know from outside the server whether something is a valid route or not, so we have to move the special favicon.ico handling (which prevents unwanted logging) out of the dev task and into the server.

maybe we should do the same thing for /service-worker.js (since that's another file that browsers like to request a lot)

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