Switch back to base Entity class, DRYer code, refs T225

Test Plan:
```
for x in tags media config; do
    bin/behat -c application/tests/behat.yml -f progress application/tests/features/api.$x.feature
done
```

Reviewers: dkobia, rjmackay

Reviewed By: rjmackay

Subscribers: lkamau, eyedol, PapaLee

Maniphest Tasks: T225

Differential Revision: https://phabricator.ushahidi.com/D104