Diff - resolve foreign key

This is a small change to the diff functions that allows "resolving" a ForeignKey field so that it displays something human-readable instead of whatever the key is.

For example, when the moderated object has a ForeignKey referencing a User, instead of the User's pk being displayed in the diff, the string representation of the User is displayed.
Instead of `1 - 4` being displayed for a change, `admin - moderator` would be displayed.