Only init emoji map once.

`emoji.init_colons` creates and populates `emoji.map.colons`, a global
map from common names to emoji code points. It's safe to call
repeatedly, but unecessary.

js/views/conversation_view.js

@@ -16,6 +16,7 @@
 (function () {
     'use strict';
     window.Whisper = window.Whisper || {};
+    emoji.init_colons();
 
     Whisper.ConversationView = Whisper.View.extend({
         className: function() {
@@ -30,8 +31,6 @@
 
             this.render();
 
-            emoji.init_colons();
-
             this.fileInput = new Whisper.FileInputView({
                 el: this.$el.find('.attachments')
             });