Move summary_adjective to its own method

(imported from commit d451ffcf3ff460ffe030e265ca83fec1094c6217)

static/js/message_list.js

@@ -282,6 +282,17 @@ MessageList.prototype = {
         }
     },
 
+    summary_adjective: function (message) {
+        if (_.contains(message.flags, 'force_collapse')) {
+            return 'collapsed';
+        } else if (!_.contains(message.flags, 'force_expand')) {
+            if (this.is_summarized_message(message)) {
+                return 'read';
+            }
+        }
+        return null;
+    },
+
     selected_idx: function MessageList_selected_idx() {
         return util.lower_bound(this._items, this._selected_id,
                                 function (a, b) { return a.id < b; });

static/js/message_list_view.js

@@ -164,15 +164,7 @@ MessageListView.prototype = {
                 finish_summary();
             }
 
-            var summary_adjective;
+            var summary_adjective = list.summary_adjective(message);
-
-            if (_.contains(message.flags, 'force_collapse')) {
-                summary_adjective = 'collapsed';
-            } else if (!_.contains(message.flags, 'force_expand')) {
-                if (list.is_summarized_message(message)) {
-                    summary_adjective = 'read';
-                }
-            }
 
             if (summary_adjective) {
                 if (prev) {