Refactor posts GET to use a Read Usecase

Summary:
Extend the generic ReadUsecase with custom loading logic
for getting posts by some id/parent/locale combo.

Update action_get_index() to use a use case and remove
custom action_get_index() in translations api.

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/posts

Reviewers: aMoniker, vladimir, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T438

Differential Revision: https://phabricator.ushahidi.com/D399