typeahead: Add pm_conversations module.

In pm_conversations.js, added function to make a user a PM partner and
another function to check if a user is a PM partner. A PM partner is
someone with whom the user has been in a PM with.

frontend_tests/node_tests/message_store.js

@@ -1,6 +1,7 @@
 add_dependencies({
     people: 'js/people.js',
     util: 'js/util.js',
+    pm_conversations: 'js/pm_conversations.js',
 });
 
 var noop = function () {};