In admin model forms, the change_form_fieldsets templatetag will

instantiate an adapter object to convert django's builtin class names

(like 'wide') to our form CSS class names ('-is-wide').
For settings forms, the template doesn't (and really can't) use that

templatetag, since a settings form is different from an adminform. In

these cases we were still using the django class names, which resulted

in using 'wide' as the class name, which wasn't correct.
This change makes our djblets_forms fieldset template check for the

'wide' class, and add '-is-wide' if found.

Viewed settings forms in the admin and saw that fieldsets now had the

correct class name, and that we now had a match between the label widths

in regular fieldsets and in subforms (which go through slightly

different templates, ugh).