Make Post Update usecase closer to the generic version

Summary:
- Make UpdatePost usecase closer to the generic usecase
- Use getEntity pattern in UpdateUsecase too
- Fix inconsitent param names in UpdateRepository vs Ushahidi_Repository
- Use entity->id not input-> to reload updated entities

Test Plan: ./bin/behat --config application/tests/behat.yml application/tests/features/api.posts.feature --tags=update

Reviewers: vladimir, aMoniker, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T532

Differential Revision: https://phabricator.ushahidi.com/D404