cmd/tailscale: refactor shared utility methods

Refactor two shared functions used by the tailscale cli,
calcAdvertiseRoutes and licensesURL. These are used by the web client as
well as other tailscale subcommands. The web client is being moved out
of the cli package, so move these two functions to new locations.

Updates tailscale/corp#13775

Signed-off-by: Will Norris <will@tailscale.com>

cmd/tailscale/depaware.txt

@@ -81,6 +81,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/hostinfo                                       from tailscale.com/net/interfaces+
         tailscale.com/ipn                                            from tailscale.com/cmd/tailscale/cli+
         tailscale.com/ipn/ipnstate                                   from tailscale.com/cmd/tailscale/cli+
+        tailscale.com/licenses                                       from tailscale.com/cmd/tailscale/cli
         tailscale.com/metrics                                        from tailscale.com/derp
         tailscale.com/net/dns/recursive                              from tailscale.com/net/dnsfallback
         tailscale.com/net/dnscache                                   from tailscale.com/derp/derphttp+