Fix using modeladmin_register as a decorator
Fix using modeladmin_register as a decorator

Using `modeladmin_register` as a decorator worked, and the ModelAdmin subclass was registered successfully, but as `modeladmin_register` did not return the ModelAdmin subclass, the decorated class would be `None`. Returning the class prevents this issue, so `modeladmin_register` will now work as expected as a decorator.
FlipperPA/wagtail,mixxorz/wagtail,thenewguy/wagtail,zerolab/wagtail,thenewguy/wagtail,rsalmaso/wagtail,wagtail/wagtail,rsalmaso/wagtail,nealtodd/wagtail,thenewguy/wagtail,mixxorz/wagtail,zerolab/wagtail,mixxorz/wagtail,jnns/wagtail,nealtodd/wagtail,FlipperPA/wagtail,takeflight/wagtail,zerolab/wagtail,nealtodd/wagtail,kaedroho/wagtail,wagtail/wagtail,rsalmaso/wagtail,jnns/wagtail,nimasmi/wagtail,jnns/wagtail,timorieber/wagtail,rsalmaso/wagtail,takeflight/wagtail,timorieber/wagtail,FlipperPA/wagtail,torchbox/wagtail,mixxorz/wagtail,mikedingjan/wagtail,kaedroho/wagtail,nealtodd/wagtail,takeflight/wagtail,wagtail/wagtail,wagtail/wagtail,takeflight/wagtail,torchbox/wagtail,kaedroho/wagtail,gasman/wagtail,gasman/wagtail,FlipperPA/wagtail,gasman/wagtail,thenewguy/wagtail,timorieber/wagtail,nimasmi/wagtail,gasman/wagtail,timorieber/wagtail,torchbox/wagtail,rsalmaso/wagtail,mikedingjan/wagtail,kaedroho/wagtail,zerolab/wagtail,nimasmi/wagtail,thenewguy/wagtail,gasman/wagtail,torchbox/wagtail,mixxorz/wagtail,nimasmi/wagtail,zerolab/wagtail,mikedingjan/wagtail,jnns/wagtail,mikedingjan/wagtail,wagtail/wagtail,kaedroho/wagtail