[Enhancement] Add default avatar when profile image is not available (#188)

* Add default avatar when agent image is not available

* Remove fonts from avatar
Separate non-computed style values

app/javascript/dashboard/components/widgets/conversation/ConversationHeader.vue

@@ -5,6 +5,7 @@
         :src="chat.meta.sender.thumbnail"
         size="40px"
         :badge="chat.meta.sender.channel"
+        :username="chat.meta.sender.name"
       />
       <h3 class="user--name">
         {{ chat.meta.sender.name }}