Spurious “invalid XSRF token” error on rename of link

[avidrissman]

To repro:

1. Go into any link editing page
2. Edit the name of the link, not the destination
3. Click the Update button

This always gives an “invalid XSRF token” error

[offbyone]

From what I can tell, that's because the save function is only allowing that token to be valid for the .new action, not the shortlink name, which is what would be needed. I'm not sure how to fix that, but I'm looking at it (note: not a Tailscale dev, just trying to read the code)