Add Post Authorizer implemented WITHOUT A2/ACL

Summary:
- Fix posts count to only include accessible posts
- Add Post Authorizer implemented WITHOUT A2/ACL

Depends on D216

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

Reviewers: eyedol, lkamau, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T494

Differential Revision: https://phabricator.ushahidi.com/D279