This is a combination of two backports, one for the new fieldsets

template, and one for adding support for Django fieldsets.
The new fieldsets template is available in

siteconfig/settings_fieldsets.html, unlike the new template which is in

forms/admin/form_fieldsets.html. The legacy location is used to

prevent breakages with users of Djblets, since it would require adding

the djblets.forms to INSTALLED_APPS.
The old template that this fieldset support came from only supported

our old custom version of fieldsets, rather than Django's version.

In Djblets 0.10, we added support for both. This change is backporting

that to the new fieldsets template.

Tested with old-style and new-style fieldsets.
Unit tests pass.