remove last posted non replied discussions

fixes COMMS-1457

Test Plan:
-Create a course
-Add a discussion
-On the discussions index page, see that a "Last Post" time
 does not appear
- add some replies
- notice the last post time appears

Change-Id: Ide93d613ab6a7c93936c59b42837845c7a6384d9
Reviewed-on: https://gerrit.instructure.com/165538
Reviewed-by: Aaron Kc Hsu <ahsu@instructure.com>
Tested-by: Jenkins
QA-Review: Landon Gilbert-Bland <lbland@instructure.com>
Product-Review: Steven Burnett <sburnett@instructure.com>

app/jsx/discussions/components/DiscussionRow.js

@@ -236,7 +236,7 @@ export class DiscussionRow extends Component {
         : ' '
     result += dueDateString
     const lastReplyAtDate = $.datetimeString(this.props.discussion.last_reply_at)
-    if (lastReplyAtDate.length > 0) {
+    if (lastReplyAtDate.length > 0 && this.props.discussion.discussion_subentry_count > 0) {
       result += I18n.t('Last post at %{date}', {date: lastReplyAtDate})
     }
     return result
@@ -595,7 +595,7 @@ export class DiscussionRow extends Component {
 
   renderLastReplyAt = () => {
     const datetimeString = $.datetimeString(this.props.discussion.last_reply_at)
-    if (!datetimeString.length) {
+    if (!datetimeString.length || this.props.discussion.discussion_subentry_count === 0) {
       return null
     }
     return (