Implement get single and get collection for media endpoint, refs T444

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

Reviewers: rjmackay, eyedol, dkobia

Reviewed By: dkobia

Subscribers: bodacea, srutto

Maniphest Tasks: T444

Differential Revision: https://phabricator.ushahidi.com/D224