Add setting to disable the sign in button

[trishume]

Designed to pair with #30444 to enable enterprises to make it harder to sign into the collab server and perhaps accidentally end up sending code to Zed.

Release Notes:

• N/A

[maan2003]

make it harder to sign into the collab server and perhaps accidentally end up sending code to Zed

I have "server_url":"https://127.0.0.1:389" in settings, so nothing can connect to zed servers even when signed out.

[trishume]

@SomeoneToIgnore it still shows the connection button, so if the user signs in via command (still possible) then it'll show their user portrait. It's only the "Sign In" button which this hides, which is annoying if you're never going to sign in.

[nathansobo]

If this is helpful, happy to add it to remove the annoyance of a button that someone would never want to click. Thanks!