🚸(frontend) mute participants by default when joining a large meeting

When joining a meeting that already has many participants, new
participants are now muted by default.

This is an empirical change, not directly requested by users but
informed by user experience: a lot of people joining large meetings
arrive with their microphone and/or camera open, which can be
painful for the host, who otherwise has to mute everyone or ask
everyone to mute. Many of these participants are inattentive but
still noisy.

If you are joining a room that is already at a certain size, you are
probably not the one expected to speak; presenters typically join
among the very first participants.

src/backend/meet/settings.py

@@ -413,7 +413,10 @@ class Base(Configuration):
         ),
         "max_participants_for_sound": values.PositiveIntegerValue(
             5, environ_name="FRONTEND_MAX_PARTICIPANTS_FOR_SOUND", environ_prefix=None
-        )
+        ),
+        "auto_mute_on_join_threshold": values.PositiveIntegerValue(
+            50, environ_name="FRONTEND_AUTO_MUTE_ON_JOIN_THRESHOLD", environ_prefix=None
+        ),
     }
 
     # Mail