Cleaned up and refactored `ModelAdmin.formfield_for_dbfield`:

  * The new method uses an admin configuration option (`formfield_overrides`); this makes custom admin widgets especially easy.
  * Refactored what was left of `formfield_for_dbfield` into a handful of smaller methods so that it's easier to hook in and return custom fields where needed.
  * These `formfield_for_*` methods now pass around `request` so that you can easily modify fields based on request (as in #3987).

Fixes #8306, #3987, #9148.

Thanks to James Bennet for the original patch; Alex Gaynor and Brian Rosner also contributed.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9760 bcc190cf-cafb-0310-a4f2-bffc1f526a37