Remove hardcoded URLs from change_form.html

This commit port the following fixes from Django to grappelli:

aaf77c1676e44019abe544911ff7a06eb2690295
by Ramiro Morales (cramm0 at gmail.com)

c86a9b63984f6692d478f6f70e3c78de4ec41814
by Loic Bistuer (loic.bistuer at sixmedia.com)

aee9eecb920cf281e8339a5f7edadc6f2dd04fea
by Daniel Hepper (daniel.hepper at gmail.com)