While Review Board 1.7.x introduced FileDiffData for new diffs, it never

gained the migration support. This was added for 2.0, by way of

on-access migration in FileDiff and the condensediffs management

command.
Diff migration has been thoroughly tested at this point, and is known to

work really well. It's also a beneficial thing to run before an upgrade

to 2.0. This change backports everything needed to get modern

(pre-RawFileDiffData) migration in 1.7.x.

Tested on-access migration on some FileDiffs containing diff data through

the shell. Saw that, prior to diff access, there was no associated FileDiffData,

but afterwards there was.
Tested that accessing diffs through the diff viewer worked and performed the migration.
Ran condensediffs and saw the progress notification and that diffs were migrated.

Verified the results.