Implements Post listing UI. Fixes T832

Summary:
 - Initial implementation of post activity
 - Display posts in card
 - Implement initial Post layout
 - Shows navigation drawer
 - shows ActionBar menus
 - Implements swipe to refresh
 - Make `List<Tag>` and `User` Transient. I need to implement converter for them.
 - Add media tag user and post models
 - Add media tag user and post model data mapper
 - Add tests for media tag user and post model data mapper
 - Add tests media tag user and post models

Todos:
 - Setup navigation drawer menu items.
 - Add a menu item for launching deployment management activity.
 - Add search bar support.
 - Fine tune UI when the final mockup is ready.

Test Plan:
 - `.gradlew test spoon`
 - Grap a build from http://qa.ushahididev.com/job/Ushahidi-Platform-Android/ws/app/build/outputs/apk/

Reviewers: dkobia, dalezak, PapaLee

Subscribers: rjmackay, shadowhand

Maniphest Tasks: T832

Differential Revision: https://phabricator.ushahidi.com/D542