Feature: View a contact's previous conversation (#422)

* Add API to fetch conversations of a contact

* Add conversation list in sidebar

app/views/api/v1/contacts/conversations/index.json.jbuilder

@@ -0,0 +1,26 @@
+json.payload do
+  json.array! @conversations do |conversation|
+    json.meta do
+      json.sender do
+        json.id conversation.contact.id
+        json.name conversation.contact.name
+        json.thumbnail conversation.contact.avatar_url
+        json.channel conversation.inbox.try(:channel_type)
+      end
+      json.assignee conversation.assignee
+    end
+
+    json.id conversation.display_id
+    if conversation.unread_incoming_messages.count.zero?
+      json.messages [conversation.messages.last.try(:push_event_data)]
+    else
+      json.messages conversation.unread_messages.map(&:push_event_data)
+    end
+    json.inbox_id conversation.inbox_id
+    json.status conversation.status_before_type_cast
+    json.timestamp conversation.messages.last.try(:created_at).try(:to_i)
+    json.user_last_seen_at conversation.user_last_seen_at.to_i
+    json.agent_last_seen_at conversation.agent_last_seen_at.to_i
+    json.unread_count conversation.unread_incoming_messages.count
+  end
+end