Add api/stats/posts, refs T98

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

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

Subscribers: srutto, sebm90

Maniphest Tasks: T98

Differential Revision: https://phabricator.ushahidi.com/D238