Add team creation disable configuration (#1802)

* Add ability for admins to disable public team creation
* Closes #1364

CTFd/teams.py

@@ -191,6 +191,12 @@ def new():
     infos = get_infos()
     errors = get_errors()
 
+    if bool(get_config("team_creation", default=True)) is False:
+        abort(
+            403,
+            description="Team creation is currently disabled. Please join an existing team.",
+        )
+
     user = get_current_user_attrs()
     if user.team_id:
         errors.append("You are already in a team. You cannot join another.")