ipn: add WebServerConfig, add views

cmd/viewer couldn't deal with that map-of-map. Add a wrapper type
instead, which also gives us a place to add future stuff.

Updates tailscale/corp#7515

Change-Id: I44a4ca1915300ea8678e5b0385056f0642ccb155
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