improve: OAuth2 UI and test suite (via #5066)

* create `features` folder

* add base oauth2 server

* continue implementing OAuth tests

* WIP

* add password flow tests

* modify Password flow credential types

* remove query string credential type

* add test case for Authorization flow

* add specific Authorization value for Password flow test

* WIP

* fix linter issues

src/core/components/live-response.jsx

@@ -80,7 +80,7 @@ export default class LiveResponse extends React.Component {
           </div>
         }
         <h4>Server response</h4>
-        <table className="responses-table">
+        <table className="responses-table live-responses-table">
           <thead>
           <tr className="responses-header">
             <td className="col col_header response-col_status">Code</td>