Update LTS support period and Django compatibility
Update LTS support period and Django compatibility

As discussed during the core team meeting of March 28th 2018.

thenewguy/wagtail,kaedroho/wagtail,jnns/wagtail,wagtail/wagtail,FlipperPA/wagtail,mixxorz/wagtail,zerolab/wagtail,FlipperPA/wagtail,mikedingjan/wagtail,torchbox/wagtail,torchbox/wagtail,timorieber/wagtail,rsalmaso/wagtail,kaedroho/wagtail,mixxorz/wagtail,mixxorz/wagtail,thenewguy/wagtail,timorieber/wagtail,kaedroho/wagtail,mikedingjan/wagtail,zerolab/wagtail,thenewguy/wagtail,timorieber/wagtail,mikedingjan/wagtail,FlipperPA/wagtail,kaedroho/wagtail,gasman/wagtail,thenewguy/wagtail,nimasmi/wagtail,mixxorz/wagtail,rsalmaso/wagtail,FlipperPA/wagtail,nealtodd/wagtail,wagtail/wagtail,zerolab/wagtail,nealtodd/wagtail,thenewguy/wagtail,gasman/wagtail,zerolab/wagtail,takeflight/wagtail,takeflight/wagtail,jnns/wagtail,wagtail/wagtail,nimasmi/wagtail,kaedroho/wagtail,rsalmaso/wagtail,mixxorz/wagtail,jnns/wagtail,gasman/wagtail,torchbox/wagtail,timorieber/wagtail,mikedingjan/wagtail,rsalmaso/wagtail,nimasmi/wagtail,zerolab/wagtail,takeflight/wagtail,wagtail/wagtail,nimasmi/wagtail,wagtail/wagtail,nealtodd/wagtail,nealtodd/wagtail,takeflight/wagtail,gasman/wagtail,rsalmaso/wagtail,torchbox/wagtail,jnns/wagtail,gasman/wagtail