Pre-populate sort title in edit book form if not provided

[mouse-reeve]

It's confusing to edit a book when this isn't set, so this provides the best-guess version of the sort title if there isn't one provided, and allows the user to change it as needed.

Fixes #2944

[mouse-reeve]

@bookwyrm-social/code-review I think this one is also pretty straightforward, if a little weird to test. On bookwyrm.social, I ended up skipping the migration that populated default sort titles, so this issue is specific to that instance. It's a little hard to replicate it because if you save a book with a blank sort title, one is populated by the save function. You can comment that out, or use models.Edition.objects.filter(id=<book_id>).update(sort_title=None) to update the field without triggering the save function.

[hughrun]

Seems to work as expected, I can't see any obvious problems other than we are checking for null values inside a check for null values.