Format API response output using Formatters, fixes T257

Summary:
- add getMimeType to Formatter interface
- add `formatter.json` and `formatter.jsonp` services
- use Formatters in API controller for output

Test Plan:
```
bin/behat -c application/tests/behat.yml -f progress application/tests/features/api.tags.feature
```

Reviewers: dkobia, rjmackay

Reviewed By: rjmackay

Subscribers: PapaLee, eyedol, lkamau

Maniphest Tasks: T257

Differential Revision: https://phabricator.ushahidi.com/D109