Add automatic migration from FileDiff to FileDiffData on access.

In 1.7, we added the FileDiffData model, which stores the data of a diff
(be it a standard diff or a parent diff) in one place, hashed, in order
to reduce file size. This was only applied to newer diffs, and older
ones still had their data retrieved from the FileDiff.diff64 or
FileDiff.parent_diff64 fields.

This change adds automatic migration to FileDiffData when accessing
either FileDiff.diff or FileDiff.parent_diff. When accessed, we check if
there's an appropriate hash for the given field, and if not, we create a
FileDiffData for both and move the data over.

This ensures that we only ever read data from one spot now. As older
review requests are accessed, the database may shrink in size (in
general, assuming there are some redundant bits of diff data).

Ensuring that there's always a FileDiffData when dealing with diff data
simplifies some of our assumptions and will allow some new pieces of common
information we'll add to exist in only one place as well.

Unit tests were added to ensure that the migration paths work as
expected, and clean up after the old data.

All unit tests pass.

Went through a bunch of my old review requests on my test server, some with
parent diffs, and saw the diffs load correctly. Went through the server logs
to make sure I saw the expected log entries.