The new template for the change forms tried to use defaults if variables

for controlling the presence of the Save buttons weren't provided.

However, it did this using |default:True, which meant that a value of

False would be turned into True. Not what we wanted.
That, however, was put in place (in change 663f50d) in order to fix a

prior (incorrect) attempt that used |default_if_none. That could have

worked if that filter considered a missing variable to be equivalent to

None, but it didn't.
Even with these attempts, we had two code paths for generating that

template: Ours, and Django's, depending on whether a change form was

being rendered.
This change implements a more future-proof, reliable attempt at

generating these buttons. We now have a dedicated template tag that

covers the logic in Django's own submit_row template tag and our own

logic, and expands upon that to allow greater customization by callers

(a Delete button is now possible without the rest of the admin classes).
With this, the Settings forms and Model Change forms all now show the

correct buttons.

Tested all the Settings forms, various model change forms, and the

integrations/extensions forms (which use their own templates anyway).