ipn/ipnlocal: start implementing web server bits of serve

Updates tailscale/corp#7515

Change-Id: I96f4016161ba3c370492da941274c6d9a234c2bb
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f