[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/ConversationCard.vue

@@ -8,6 +8,8 @@
       :src="chat.meta.sender.thumbnail"
       :badge="chat.meta.sender.channel"
       class="columns"
+      :username="chat.meta.sender.name"
+      size="40px"
     />
     <div class="conversation--details columns">
       <h4 class="conversation--user">