Pholio - add concept of replacing images and primitive history view

Summary:
Now you can actually replace an image! Ref T3572. This ended up needing a wee bit of infrastructure to work...

 - add replace image transaction to pholio
 - add replacesImagePHID to PholioImage
 - tweaks to editor to properly update images with respect to replacement
   - add edges to track replacement
 - expose getNodes on graph query infrastructure to query the entire graph of who replaced who
 - move pholio image to new phid infrastructure

Still TODO - the history view should get chopped out a bit from the current view - no more inline comments / generally less functionality plus maybe a tweak or two to make this more sensical.

Test Plan: replaced images and played with history controller a little. works okay.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3572

Differential Revision: https://secure.phabricator.com/D6560