refactor: Rename MessageList.muting_enabled.

Previously, the `muting_enabled` property of
MessageListData class was used to indicate whether
some messages in the message list need to be
filtered due to topic muting, depending on the
narrow. For example, we exclude messages belonging
to muted topics from stream narrows, but not from
search narrows.

The name `muting_enabled` is a bit confusing, and hence is
changed to `excludes_muted_topics`.
It is also important that the name be specific, since
a similar new property will be added for user mutes
in future commits.

static/js/message_fetch.js

@@ -466,7 +466,7 @@ exports.initialize = function () {
     // without a `table_name` attached.
     const recent_topics_message_list = new message_list.MessageList({
         filter: new Filter([{operator: "in", operand: "home"}]),
-        muting_enabled: true,
+        excludes_muted_topics: true,
     });
     exports.load_messages({
         anchor: "newest",