R1:f75f3a0c3b32

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…