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

* Add API to fetch conversations of a contact

* Add conversation list in sidebar

app/javascript/dashboard/api/contacts.js

@@ -1,9 +1,14 @@
+/* global axios */
 import ApiClient from './ApiClient';
 
 class ContactAPI extends ApiClient {
   constructor() {
     super('contacts');
   }
+
+  getConversations(contactId) {
+    return axios.get(`${this.url}/${contactId}/conversations`);
+  }
 }
 
 export default new ContactAPI();