Implement Endpoint within the media API, refs T696

Summary:
- add usecase.media.read and usecase.media.search
- integrate authorizer with all usecases
- endpoint and authorizer cleanup

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

Reviewers: aMoniker, rjmackay

Reviewed By: rjmackay

Subscribers: srutto, bodacea

Maniphest Tasks: T696

Differential Revision: https://phabricator.ushahidi.com/D334