Implement post data repository. Fixes T1114

Summary:
 - Add post entity
 - Add post entity mapper
 - Add post database helper
 - Refactor post entity in core to get value to return a string instead
 - Add post database helper
 - Implement validation for post title and content
 - Add test for post entity
 - Add test for post entity mapper
 - Add test for post database helper
 - Add missing property to post entity

Test Plan:
Run
 - `./gradlew test`

Reviewers: dkobia, dalezak, PapaLee

Subscribers: shadowhand, rjmackay

Maniphest Tasks: T1114

Differential Revision: https://phabricator.ushahidi.com/D529