Add PostValue entity and repositories

Summary:
Post value entity and repositories replace the functions of the Post_* ORM models.

- Add post value entity and spec
- Add PostValueRepository
- Add PostValue repository implementations, 1 for each attribute type

Refs T438

Test Plan:
`./bin/phpspec run spec/Ushahidi/Entity/PostValueSpec.php `
`./bin/phpunit -c application/tests/phpunit.xml application/tests/classes/Ushahidi/PostValueTest.php`

Reviewers: dkobia, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T438

Differential Revision: https://phabricator.ushahidi.com/D222