Implement media delete with use case, refs T444

Summary: - includes the possibility of ownership checks, but does not use them

Test Plan:
```
bin/phpspec run spec/Ushahidi/Usecase/Media/DeleteSpec.php
bin/behat -f progress -c application/tests/behat.yml application/tests/features/api.media.feature
```

Reviewers: eyedol, dkobia, rjmackay

Reviewed By: rjmackay

Subscribers: bodacea, srutto

Maniphest Tasks: T444

Differential Revision: https://phabricator.ushahidi.com/D225