Add Undo functionality

[obenland]

Fixes #1289

Proposed changes:

• Moves get_activity() and get_author() methods to Outbox class, where they feel more appropriate. These methods take an outbox id.
• Adds undo() method to Outbox class, that accepts an outbox item and creates an Undo activity based on it. It accounts for Create and Add activities requiring their own "undo" type.
• Updates Base::transform_object_properties() and Base_Object::to_array() to not only discard null values but also empty values unless they're false.
• Adds unit tests for the new functionality.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• It's not hooked in anywhere, so tests are probably most appropriate at this point?
• The changes to Base::transform_object_properties() and Base_Object::to_array() are probably what has the highest potential for unintended consequences. Anything standing you there?

[obenland]

Tests fail because the activity saved in Outbox items has tag and cc properties, and the Activity object doesn't?

[obenland]

@pfefferle Setting the default for cc to null actually results in it showing up in objects as cc:[null] since there are no more checks after set_audience() runs.

Not sure how you feel about defaulting to an empty array for it in Activity?

[obenland]

New approach: Being more prescriptive than checking for isset() also removes these empty properties.

[mattwiebe]

This seems good to me.

Where, if anywhere, could we also expose this in the UI? Should we add a wp-cli command?

[obenland]

I like that, yes!