Add pm_conversations.recent.get_strings().

We will use this method for hotkey navigation through recent
pms.

frontend_tests/node_tests/pm_conversations.js

@@ -25,5 +25,7 @@ var pmc = zrequire('pm_conversations');
         {user_ids_string: '1', timestamp: 3001},
         {user_ids_string: '2', timestamp: 2001},
     ]);
+
+    assert.deepEqual(pmc.recent.get_strings(), ['1', '2']);
 }());
 

static/js/pm_conversations.js

@@ -51,6 +51,12 @@ exports.recent = (function () {
         return recent_private_messages;
     };
 
+    self.get_strings = function () {
+        // returns array of structs with user_ids_string and
+        // timestamp
+        return _.pluck(recent_private_messages, 'user_ids_string');
+    };
+
     return self;
 }());