hotspots: Add TUTORIAL_ENABLED setting to toggle INTRO_HOTSPOTS.

We add a TUTORIAL_ENABLED setting for self-hosters who want to
disable the tutorial entirely on their system. For this, the
default value (True) is placed in default_settings.py, which
can be overwritten by adding an entry in /etc/zulip/settings.py.

zerver/lib/hotspots.py

@@ -71,6 +71,10 @@ def get_next_hotspots(user: UserProfile) -> List[Dict[str, object]]:
             for hotspot in ALL_HOTSPOTS
         ]
 
+    # If a Zulip server has disabled the tutorial, never send hotspots.
+    if not settings.TUTORIAL_ENABLED:
+        return []
+
     if user.tutorial_status == UserProfile.TUTORIAL_FINISHED:
         return []