OAuth 2.0 dance for 3 grant_types with sample login app

### Commit [90a64f4]
1. Clean up the oauth-login-app from the branch [ remotes/upstream/oauth-login ](https://github.com/apigee/api-platform-samples/tree/oauth-login/sample-proxies/oauth-login-app)
2. Add invoke.sh script for easy understanding. 

### Commit [11c30ac]
1. Remove unnecessary policies
2. Simplify the conditional flows as the conditions will be handled by the policies itself. 
3. Format the xml snippets