Implementation of tags collection API through use case, refs T238

Summary: Implements the tag collection endpoint using the CRUD interface of the Collection use case.

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

Reviewers: rjmackay, dkobia

Reviewed By: dkobia

Subscribers: PapaLee, lkamau, eyedol

Maniphest Tasks: T238

Differential Revision: https://phabricator.ushahidi.com/D99