FEATURE: show posters as opposed to participants in messages

Inbox now mixes stuff that you started with stuff you replied to, removing
your avatar from the avatar list causes a lot of confusion.

app/assets/javascripts/discourse/controllers/user-topics-list.js.es6

@@ -2,7 +2,7 @@
 export default Ember.Controller.extend({
   needs: ["application", "user"],
   hideCategory: false,
-  showParticipants: false,
+  showPosters: false,
 
   _showFooter: function() {
     this.set("controllers.application.showFooter", !this.get("model.canLoadMore"));

app/assets/javascripts/discourse/routes/build-private-messages-route.js.es6

@@ -21,7 +21,7 @@ export default (viewName, path) => {
 
       this.controllerFor("user-topics-list").setProperties({
         hideCategory: true,
-        showParticipants: true,
+        showPosters: true,
         canBulkSelect: true,
         selected: []
       });

app/assets/javascripts/discourse/templates/components/basic-topic-list.hbs

@@ -2,6 +2,7 @@
   {{#if topics}}
     {{topic-list
         showParticipants=showParticipants
+        showPosters=showPosters
         hideCategory=hideCategory
         topics=topics
         expandExcerpts=expandExcerpts

app/assets/javascripts/discourse/templates/user-topics-list.hbs

@@ -2,6 +2,7 @@
 {{basic-topic-list topicList=model
                    hideCategory=hideCategory
                    showParticipants=showParticipants
+                   showPosters=showPosters
                    canBulkSelect=canBulkSelect
                    bulkSelectEnabled=bulkSelectEnabled
                    selected=selected