The new change form was using default_if_none to specify the defaults

for visibility of various buttons, but that doesn't do the right thing

if the variables being checked just aren't defined at all in the

context. This change fixes it to use default instead, and enables

visibility of the "Save and add another" and "Save and continue editing"

by default.

Opened the admin form for creating new objects (repositories, users,

groups) and saw that the submit row buttons were now visible.