Implement posts create use case

Summary:
- Make action_post_index_collection use usecase. Scaffold parser+validator
- Remove old code for saving posts
- Implement PostRepo->create()
- UpdatePostData -> PostData

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

Reviewers: aMoniker, rjmackay, vladimir

Reviewed By: vladimir

Maniphest Tasks: T534

Differential Revision: https://phabricator.ushahidi.com/D405