refactor: Cleanly divide data/view for add_messages.

Before this change, the way to add messages had a lot
of ping-pong-ing between MessageList and MessageListData,
where first the data got triaged, but not actually
inserted into data structures, and then subsequent
calls would add the data and get filtered results.

Now we have a simple API for MessageListData.add_messages
that does all the data stuff up front.  Having a fully
function MLD.add_messages not only makes the ML.add_messages
function about four lines shorter, it also sets us up
to easily build standalone MLD objects before making
the heavier ML objects.

static/js/message_list_data.js

@@ -163,7 +163,7 @@ MessageListData.prototype = {
         });
     },
 
-    triage_messages: function (messages) {
+    add_messages: function (messages) {
         var self = this;
         var top_messages = [];
         var bottom_messages = [];
@@ -197,15 +197,37 @@ MessageListData.prototype = {
             });
         }
 
-        return {
+        // We return probably more info than our actual callers
+        // need, but this is useful for tests and not actually
+        // expensive.
+        var info = {
             top_messages: top_messages,
             bottom_messages: bottom_messages,
             interior_messages: interior_messages,
         };
+
+        if (interior_messages.length > 0) {
+            var all_messages = top_messages.concat(interior_messages).concat(bottom_messages);
+            self.add_anywhere(all_messages);
+            return info;
+        }
+
+        if (top_messages.length > 0) {
+            top_messages = self.prepend(top_messages);
+        }
+
+        if (bottom_messages.length > 0) {
+            bottom_messages = self.append(bottom_messages);
+        }
+
+        return info;
     },
 
-    add: function (messages) {
-        // Caller should have already filtered
+    add_anywhere: function (messages) {
+        // Caller should have already filtered messages.
+        // This should be used internally when we have
+        // "interior" messages to add and can't optimize
+        // things by only doing prepend or only doing append.
         var viewable_messages;
         if (this.muting_enabled) {
             this._all_items = messages.concat(this._all_items);