Internationalize open/close chat messages

Per
https://edx-wiki.atlassian.net/wiki/pages/viewpage.action?spaceKey=ENG&title=I18n+coding+guidelines
ensure that chat messages are properly internationalized.

lms/templates/courseware/courseware.html

 <%inherit file="/main.html" />
 <%namespace name='static' file='/static_content.html'/>
+<%! from django.utils.translation import ugettext as _ %>
 <%block name="bodyclass">courseware ${course.css_class}</%block>
 <%block name="title"><title>${course.number} Courseware</title></%block>
 
@@ -192,8 +193,8 @@
 % if show_chat:
   <div id="chat-wrapper">
     <div id="chat-toggle" class="closed">
-      <span id="chat-open">Open Chat <em class="icon-chevron-up"></em></span>
-      <span id="chat-close">Close Chat <em class="icon-chevron-down"></em></span>
+      <span id="chat-open">${_("Open Chat")} <em class="icon-chevron-up"></em></span>
+      <span id="chat-close">${_("Close Chat")} <em class="icon-chevron-down"></em></span>
     </div>
     <div id="chat-block">
       ## The Candy.js plugin wants to render in an element with #candy