Quickly switch between Tailscale accounts

This is a big deal. People have been wanting some way to simultaneously use multiple tailnets for a while (https://github.com/tailscale/tailscale/issues/183), which makes all the sense when you consider that the customer acquisition flow seems to be through individuals getting it for free and persuading their orgs to buy it.

While this is not the full "multiple tailnets at the same time" experience, this is totally good enough for me.

While it's relieving to see that it's finally here, I also wonder how it could have taken so long. How complex was it to implement? I totally understand that there's other work to do, but not doing this for so long likely cost real money.