API: Abstract Serializers tests

[deandrebaker]

We need to add tests for the following:

• AbstractSharedItemSerializer.get_is_liked returns true if the current user has liked the shared item, or false otherwise.
• AbstractSharedItemSerializer.get_is_saved returns true if the current user has saved the shared item, or false otherwise.
• AbstractSharedItemSerializer.get_likes returns the number of likes a shared item has.
• AbstractPostSerializer.get_comments returns the number of comments a shared item has.
• AbstractModelSerializer.create returns a shared item object and calls the create_tagged_users_notification with the object as an argument.
• AbstractCreatorSerializer.create returns a shared item object with the creator set as the current user.