Switch OAuth backend, fixes T285

Summary:
https://github.com/thephpleague/oauth2-server/wiki/Developing-an-OAuth-2.0-authorization-server
https://github.com/thephpleague/oauth2-server/wiki/Securing-your-API-with-OAuth-2.0

- uninstall `bshaffer/oauth2-server-php` from composer
- remove `modules/koauth` from Kohana
- install `league/oauth2-server` with composer, implement storage, inject deps
- update Base.yml and tests
- remove `jso2` from JS
- write custom OAuth AJAX handler

Test Plan:
```
composer update
./minion --task=migrations:run --up
bin/behat -c application/tests/behat.yml application/tests/features/api.oauth.feature
bin/behat -c application/tests/behat.yml application/tests/features/api.posts.feature
```

See also https://travis-ci.org/ushahidi/Lamu/jobs/26344755

Reviewers: dkobia, rjmackay

Reviewed By: rjmackay

Subscribers: eyedol

Maniphest Tasks: T285

Differential Revision: https://phabricator.ushahidi.com/D120