Implement tag delete with use case, refs T358

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

Reviewers: rjmackay, eyedol, dkobia

Reviewed By: dkobia

Subscribers: bodacea, srutto

Maniphest Tasks: T358

Differential Revision: https://phabricator.ushahidi.com/D227