Upgrade and compose the OAuth2 vendor server, refs T194

Summary:
- switch `bshaffer/oauth2-server-php` to composer module
- update to latest stable verison of OAuth2 server
- update app code to match namespaced server

Test Plan:
```
bin/behat --config application/tests/behat.yml application/tests/features/api.oauth.feature --format progress
```

and logout, login, try app.

Reviewers: rjmackay, dkobia

Reviewed By: dkobia

Subscribers: lkamau, eyedol

Maniphest Tasks: T194

Differential Revision: https://phabricator.ushahidi.com/D93