Add Tag entity and repository, fixes T239

Test Plan:
```
bin/phpspec run spec/Ushahidi/Entity/TagSpec.php
```

Reviewers: dkobia, rjmackay

Reviewed By: rjmackay

Subscribers: PapaLee, eyedol, lkamau

Maniphest Tasks: T239

Differential Revision: https://phabricator.ushahidi.com/D98