Fix recent regressions with All Messages (muted topics).

My recent refactoring that split out MessageListData
introduced a nasty bug where we were putting muted
messages into the "All Messages" view even though
the underlying list was correctly filtering
them, so the symptoms were two-fold:

    - muted messages cluttered up your feed
    - replying to the message caused a traceback (since
      it wasn't actually in the underlying data
      structure)

This has to do with what MessageListData.add_messages()
was passing back to MessageList to orchestrate drawing
in MessageListView.

I think what happened here is I got this working kind
of sloppily but correctly for the non-muting case and
then got in the weeds of some other stuff.  Not my
finest moment.

The actual correct code here is simple enough.  We
triage top, interior, and bottom, and then the respective
methods that put the data into the data structure
return the filtered lists (i.e. not muted) and put them
into the info structure.

Fixes #9656

frontend_tests/node_tests/message_list_data.js

@@ -144,7 +144,16 @@ run_test('more muting', () => {
         {id: 8},
     ];
 
-    mld.add_messages(orig_messages);
+    const orig_info = mld.add_messages(orig_messages);
+
+    assert.deepEqual(orig_info, {
+        top_messages: [],
+        interior_messages: [],
+        bottom_messages: [
+            {id: 4},
+            {id: 8},
+        ],
+    });
 
     assert.deepEqual(
         _.pluck(mld._all_items, 'id'),
@@ -165,7 +174,8 @@ run_test('more muting', () => {
         {id: 9, subject: 'muted'},
         {id: 10},
     ];
-    mld.add_messages(more_messages);
+
+    const more_info = mld.add_messages(more_messages);
 
     assert.deepEqual(
         _.pluck(mld._all_items, 'id'),
@@ -177,6 +187,18 @@ run_test('more muting', () => {
         [2, 4, 6, 8, 10]
     );
 
+    assert.deepEqual(more_info, {
+        top_messages: [
+            {id: 2},
+        ],
+        interior_messages: [
+            {id: 6},
+        ],
+        bottom_messages: [
+            {id: 10},
+        ],
+    });
+
 });
 
 run_test('errors', () => {

static/js/message_list_data.js

@@ -254,19 +254,8 @@ MessageListData.prototype = {
             });
         }
 
-        // 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;
+            interior_messages = self.add_anywhere(interior_messages);
         }
 
         if (top_messages.length > 0) {
@@ -277,6 +266,12 @@ MessageListData.prototype = {
             bottom_messages = self.append(bottom_messages);
         }
 
+        var info = {
+            top_messages: top_messages,
+            bottom_messages: bottom_messages,
+            interior_messages: interior_messages,
+        };
+
         return info;
     },
 
@@ -294,6 +289,7 @@ MessageListData.prototype = {
             this._items = viewable_messages.concat(this._items);
 
         } else {
+            viewable_messages = messages;
             this._items = messages.concat(this._items);
         }