Fix crazy load times for DiffSet admin page loads.
Fix crazy load times for DiffSet admin page loads.

The `legacy_diff_hash` and `legacy_parent_diff_hash` fields in the `FileDiff`
inlines for the `DiffSet` admin pages were loading as `<select>` fields, which
hammered the database and took a long time. These should be raw ID fields
instead.

Testing done:
Looked at a `DiffSet` in the admin and saw it load very quickly.

Reviewed at https://reviews.reviewboard.org/r/7870/

chipx86/reviewboard,brennie/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,brennie/reviewboard,sgallagher/reviewboard,davidt/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,davidt/reviewboard,brennie/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,chipx86/reviewboard,brennie/reviewboard,davidt/reviewboard,davidt/reviewboard