Get EditHandler form class from model if not overridden
Get EditHandler form class from model if not overridden

Unless overridden by passing `base_form_class=CustomFormClass` to an
EditHandler, the EditHandler now gets the base form class from the
model. People who override the EditHandler of a model no longer have to
also override the base_form_class if the model needs a custom one.

Fixes #2267

chrxr/wagtail,Toshakins/wagtail,nutztherookie/wagtail,thenewguy/wagtail,kurtw/wagtail,Toshakins/wagtail,nimasmi/wagtail,hamsterbacke23/wagtail,nealtodd/wagtail,mikedingjan/wagtail,FlipperPA/wagtail,Toshakins/wagtail,kaedroho/wagtail,kurtw/wagtail,chrxr/wagtail,timorieber/wagtail,rsalmaso/wagtail,mixxorz/wagtail,thenewguy/wagtail,torchbox/wagtail,FlipperPA/wagtail,gasman/wagtail,thenewguy/wagtail,mikedingjan/wagtail,nutztherookie/wagtail,kaedroho/wagtail,thenewguy/wagtail,timorieber/wagtail,wagtail/wagtail,thenewguy/wagtail,takeflight/wagtail,kaedroho/wagtail,chrxr/wagtail,zerolab/wagtail,wagtail/wagtail,nilnvoid/wagtail,iansprice/wagtail,quru/wagtail,nutztherookie/wagtail,gasman/wagtail,nimasmi/wagtail,kaedroho/wagtail,hamsterbacke23/wagtail,nimasmi/wagtail,timorieber/wagtail,zerolab/wagtail,hamsterbacke23/wagtail,kurtw/wagtail,nutztherookie/wagtail,gasman/wagtail,kurtrwall/wagtail,timorieber/wagtail,rsalmaso/wagtail,torchbox/wagtail,gasman/wagtail,nilnvoid/wagtail,torchbox/wagtail,takeflight/wagtail,nealtodd/wagtail,zerolab/wagtail,quru/wagtail,wagtail/wagtail,torchbox/wagtail,zerolab/wagtail,kurtrwall/wagtail,wagtail/wagtail,nilnvoid/wagtail,iansprice/wagtail,jnns/wagtail,takeflight/wagtail,Toshakins/wagtail,zerolab/wagtail,nealtodd/wagtail,quru/wagtail,mixxorz/wagtail,kurtrwall/wagtail,nealtodd/wagtail,jnns/wagtail,hamsterbacke23/wagtail,mixxorz/wagtail,FlipperPA/wagtail,rsalmaso/wagtail,iansprice/wagtail,gasman/wagtail,kurtrwall/wagtail,mixxorz/wagtail,chrxr/wagtail,mikedingjan/wagtail,rsalmaso/wagtail,wagtail/wagtail,rsalmaso/wagtail,iansprice/wagtail,FlipperPA/wagtail,mixxorz/wagtail,mikedingjan/wagtail,jnns/wagtail,quru/wagtail,kurtw/wagtail,nimasmi/wagtail,kaedroho/wagtail,takeflight/wagtail,jnns/wagtail,nilnvoid/wagtail