Move getCollection to CollectionLoader trait, fixes T1103

Summary:
Makes it possible to use `getCollection` in repositories that do not extend
`Ushahidi_Repository`.

Test Plan:
```
bin/behat -c application/tests/behat.yml -f progress
```

Reviewers: aMoniker

Reviewed By: aMoniker

Subscribers: vladimir, rjmackay

Maniphest Tasks: T1103

Differential Revision: https://phabricator.ushahidi.com/D494