Refactor posts delete to repo+usecase

Summary:
- Extend delete use case for translation loading
- Convert controller to use usecase

Test Plan: ./bin/behat -c application/tests/behat.yml application/tests/features/api.posts.feature

Reviewers: vladimir, aMoniker, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T544

Differential Revision: https://phabricator.ushahidi.com/D400