Implement Media Data repository. Fixes T1080

Summary:
 - Add media entity

 - Implement entity mapper

 - Add a media not found exception

 - Add a media not found exception

 - Add media database helper

 - Add media repository

 - Add media database helper class

 - Add test for media entity

 - Add test for media repository testing

Test Plan:
Issue
 - `./gradlew test`

Reviewers: dalezak, dkobia, shadowhand

Reviewed By: shadowhand

Subscribers: rjmackay, shadowhand

Maniphest Tasks: T1080

Differential Revision: https://phabricator.ushahidi.com/D517