Add setting to disable the sign in button (#30450)

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

Co-authored-by: Mikayla Maki <mikayla@zed.dev>

assets/settings/default.json

@@ -335,7 +335,9 @@
     // Whether to show onboarding banners in the titlebar.
     "show_onboarding_banner": true,
     // Whether to show user picture in the titlebar.
-    "show_user_picture": true
+    "show_user_picture": true,
+    // Whether to show the sign in button in the titlebar.
+    "show_sign_in": true
   },
   // Scrollbar related settings
   "scrollbar": {