Update posts API to use Endpoint

Summary:
- Use Endpoint for posts get, delete and search
- Return total_count for search usecases

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

Reviewers: aMoniker, vladimir, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T564

Differential Revision: https://phabricator.ushahidi.com/D544