Complete clean code for Tags action_get_index, refs T358

Summary:
- load entity with repo
- output with formatter

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

Reviewers: dkobia, eyedol, rjmackay

Reviewed By: rjmackay

Subscribers: srutto

Maniphest Tasks: T358

Differential Revision: https://phabricator.ushahidi.com/D183