client/web: handle login client inside an iframe

If the login client is inside an iframe, open the management client in a
new window, since it can't be loaded in the frame.

Updates #10261

Signed-off-by: Will Norris <will@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f