ipn/ipnlocal,client/web: add web client to tailscaled

Allows for serving the web interface from tailscaled, with the
ability to start and stop the server via localapi endpoints
(/web/start and /web/stop).

This will be used to run the new full management web client,
which will only be accessible over Tailscale (with an extra auth
check step over noise) from the daemon. This switch also allows
us to run the web interface as a long-lived service in environments
where the CLI version is restricted to CGI, allowing us to manage
certain auth state in memory.

ipn/ipnlocal/web is stubbed out in ipn/ipnlocal/web_stub for
ios builds to satisfy ios restriction from adding "text/template"
and "html/template" dependencies.

Updates tailscale/corp#14335

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

tstest/integration/tailscaled_deps_test_darwin.go

@@ -11,6 +11,7 @@ import (
 	// transitive deps when we run "go install tailscaled" in a child
 	// process and can cache a prior success when a dependency changes.
 	_ "tailscale.com/chirp"
+	_ "tailscale.com/client/tailscale"
 	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"

tstest/integration/tailscaled_deps_test_freebsd.go

@@ -11,6 +11,7 @@ import (
 	// transitive deps when we run "go install tailscaled" in a child
 	// process and can cache a prior success when a dependency changes.
 	_ "tailscale.com/chirp"
+	_ "tailscale.com/client/tailscale"
 	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"

tstest/integration/tailscaled_deps_test_linux.go

@@ -11,6 +11,7 @@ import (
 	// transitive deps when we run "go install tailscaled" in a child
 	// process and can cache a prior success when a dependency changes.
 	_ "tailscale.com/chirp"
+	_ "tailscale.com/client/tailscale"
 	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"

tstest/integration/tailscaled_deps_test_openbsd.go

@@ -11,6 +11,7 @@ import (
 	// transitive deps when we run "go install tailscaled" in a child
 	// process and can cache a prior success when a dependency changes.
 	_ "tailscale.com/chirp"
+	_ "tailscale.com/client/tailscale"
 	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"

tstest/integration/tailscaled_deps_test_windows.go

@@ -18,6 +18,7 @@ import (
 	_ "golang.org/x/sys/windows/svc/mgr"
 	_ "golang.zx2c4.com/wintun"
 	_ "golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
+	_ "tailscale.com/client/tailscale"
 	_ "tailscale.com/cmd/tailscaled/childproc"
 	_ "tailscale.com/control/controlclient"
 	_ "tailscale.com/derp/derphttp"