[Refactor] Cleanup agent store and actions (#373)

* Cleanup agent store and actions

* Move set/create/update/destroy to helpers

* Update mutation specs

* Add specs for API helper

* Fix edit/delete action visibility

* Add actions specs

* Remove unused API helpers

* Remove duplicates

* Remove duplicates

* Fix duplicate

app/javascript/dashboard/api/agents.js

@@ -0,0 +1,9 @@
+import ApiClient from './ApiClient';
+
+class Agents extends ApiClient {
+  constructor() {
+    super('agents');
+  }
+}
+
+export default new Agents();