Add setting to enable/disable chat site-wide

To protect against chat blowing things up, we include a way to
enable and disable it on an entire site-wide basis.

lms/envs/common.py

@@ -144,6 +144,10 @@ MITX_FEATURES = {
 
     # Allow use of the hint managment instructor view.
     'ENABLE_HINTER_INSTRUCTOR_VIEW': False,
+
+    # Toggle to enable chat availability (configured on a per-course
+    # basis in Studio)
+    'ENABLE_CHAT': False
 }
 
 # Used for A/B testing