ParsedDiffFile and FileDiff now contain storage of old and new UNIX

file modes, which may be provided by the diff parser. These are

ultimately stored in FileDiff.extra_data as old_unix_mode and

new_unix_mode.
Right now, nothing consumes these within Review Board, but clients will

be able to make use of these to reflect file modes. In time, the plan is

to show these in the diff viewer.

All unit tests pass.