RB.DiffReviewable instances are built based on a RB.DiffFile, and

some of the attributes from the file end up in the reviewable. Some

upcoming code needs access to more of the file's attributes, so instead

of plumbing all those in, we now attach the file to the reviewable as an

attribute.
The old fileIndex attribute on the reviewable has been removed.

Callers now access this from the associated file.

Unit tests pass.
Manually tested navigating between revisions, pages, and file anchors

using the index.