Update AbstractForm to use mode_name for preview mode
Update AbstractForm to use mode_name for preview mode

- clean up to align with other usage across Wagtail

kaedroho/wagtail,takeflight/wagtail,jnns/wagtail,rsalmaso/wagtail,zerolab/wagtail,kaedroho/wagtail,wagtail/wagtail,torchbox/wagtail,jnns/wagtail,mixxorz/wagtail,mixxorz/wagtail,zerolab/wagtail,thenewguy/wagtail,wagtail/wagtail,zerolab/wagtail,gasman/wagtail,torchbox/wagtail,takeflight/wagtail,kaedroho/wagtail,takeflight/wagtail,FlipperPA/wagtail,jnns/wagtail,thenewguy/wagtail,gasman/wagtail,gasman/wagtail,wagtail/wagtail,FlipperPA/wagtail,thenewguy/wagtail,wagtail/wagtail,rsalmaso/wagtail,gasman/wagtail,zerolab/wagtail,FlipperPA/wagtail,mixxorz/wagtail,kaedroho/wagtail,mixxorz/wagtail,mixxorz/wagtail,thenewguy/wagtail,rsalmaso/wagtail,jnns/wagtail,kaedroho/wagtail,FlipperPA/wagtail,torchbox/wagtail,thenewguy/wagtail,takeflight/wagtail,zerolab/wagtail,wagtail/wagtail,torchbox/wagtail,rsalmaso/wagtail,gasman/wagtail,rsalmaso/wagtail