Post model cleanup

 * Posts are now loaded from the database with the passed in local Id. Removed magic numbers for db columns.
 * Removed all database work from the Post model. If the post isn't found, null is returned.
 * Removed old migrateDrafts method form WordPressDB, no longer needed.
 * Fixed camelCasing of Post getters and setters, and added a few null protections for string members.
 * Added some basic test cases for the Post model.

Fixes #968