Implement field validation for Site.is_default_site; resolves #289
Implement field validation for Site.is_default_site; resolves #289

kaedroho/wagtail,iho/wagtail,tangentlabs/wagtail,FlipperPA/wagtail,m-sanders/wagtail,chrxr/wagtail,zerolab/wagtail,nrsimha/wagtail,gogobook/wagtail,rjsproxy/wagtail,nealtodd/wagtail,mikedingjan/wagtail,jorge-marques/wagtail,iansprice/wagtail,mephizzle/wagtail,Tivix/wagtail,bjesus/wagtail,mixxorz/wagtail,tangentlabs/wagtail,torchbox/wagtail,Klaudit/wagtail,dresiu/wagtail,lojack/wagtail,stevenewey/wagtail,nilnvoid/wagtail,nealtodd/wagtail,benjaoming/wagtail,takeflight/wagtail,iansprice/wagtail,janusnic/wagtail,mikedingjan/wagtail,marctc/wagtail,chrxr/wagtail,jnns/wagtail,inonit/wagtail,nilnvoid/wagtail,rsalmaso/wagtail,iho/wagtail,m-sanders/wagtail,thenewguy/wagtail,kurtw/wagtail,kurtrwall/wagtail,nimasmi/wagtail,mayapurmedia/wagtail,m-sanders/wagtail,benjaoming/wagtail,Klaudit/wagtail,marctc/wagtail,hamsterbacke23/wagtail,inonit/wagtail,jorge-marques/wagtail,chimeno/wagtail,Toshakins/wagtail,kurtw/wagtail,takeflight/wagtail,rv816/wagtail,JoshBarr/wagtail,taedori81/wagtail,Toshakins/wagtail,takeflight/wagtail,nutztherookie/wagtail,bjesus/wagtail,mixxorz/wagtail,wagtail/wagtail,gogobook/wagtail,nrsimha/wagtail,mephizzle/wagtail,jordij/wagtail,100Shapes/wagtail,hanpama/wagtail,darith27/wagtail,Pennebaker/wagtail,jorge-marques/wagtail,KimGlazebrook/wagtail-experiment,torchbox/wagtail,nealtodd/wagtail,mikedingjan/wagtail,quru/wagtail,hanpama/wagtail,nutztherookie/wagtail,takeshineshiro/wagtail,timorieber/wagtail,nilnvoid/wagtail,chimeno/wagtail,chimeno/wagtail,mayapurmedia/wagtail,benemery/wagtail,takeshineshiro/wagtail,takeshineshiro/wagtail,JoshBarr/wagtail,hanpama/wagtail,gogobook/wagtail,stevenewey/wagtail,FlipperPA/wagtail,kurtrwall/wagtail,gasman/wagtail,kaedroho/wagtail,quru/wagtail,taedori81/wagtail,rsalmaso/wagtail,mjec/wagtail,taedori81/wagtail,chrxr/wagtail,FlipperPA/wagtail,wagtail/wagtail,thenewguy/wagtail,davecranwell/wagtail,kaedroho/wagtail,nimasmi/wagtail,hamsterbacke23/wagtail,kurtw/wagtail,taedori81/wagtail,darith27/wagtail,rv816/wagtail,zerolab/wagtail,benemery/wagtail,takeflight/wagtail,WQuanfeng/wagtail,rv816/wagtail,wagtail/wagtail,rsalmaso/wagtail,WQuanfeng/wagtail,timorieber/wagtail,nutztherookie/wagtail,mixxorz/wagtail,bjesus/wagtail,benjaoming/wagtail,serzans/wagtail,jnns/wagtail,jorge-marques/wagtail,JoshBarr/wagtail,mikedingjan/wagtail,benjaoming/wagtail,dresiu/wagtail,hamsterbacke23/wagtail,kaedroho/wagtail,Tivix/wagtail,mjec/wagtail,stevenewey/wagtail,rsalmaso/wagtail,Pennebaker/wagtail,inonit/wagtail,janusnic/wagtail,lojack/wagtail,Tivix/wagtail,Toshakins/wagtail,iho/wagtail,jordij/wagtail,mjec/wagtail,mephizzle/wagtail,kaedroho/wagtail,mixxorz/wagtail,nilnvoid/wagtail,inonit/wagtail,willcodefortea/wagtail,Pennebaker/wagtail,willcodefortea/wagtail,nealtodd/wagtail,wagtail/wagtail,nrsimha/wagtail,takeshineshiro/wagtail,FlipperPA/wagtail,m-sanders/wagtail,rjsproxy/wagtail,WQuanfeng/wagtail,timorieber/wagtail,jordij/wagtail,hanpama/wagtail,quru/wagtail,torchbox/wagtail,benemery/wagtail,100Shapes/wagtail,davecranwell/wagtail,jordij/wagtail,dresiu/wagtail,gasman/wagtail,mjec/wagtail,mayapurmedia/wagtail,serzans/wagtail,mixxorz/wagtail,nimasmi/wagtail,zerolab/wagtail,timorieber/wagtail,kurtrwall/wagtail,jorge-marques/wagtail,rjsproxy/wagtail,jnns/wagtail,thenewguy/wagtail,Tivix/wagtail,Pennebaker/wagtail,thenewguy/wagtail,rjsproxy/wagtail,marctc/wagtail,chrxr/wagtail,gasman/wagtail,marctc/wagtail,hamsterbacke23/wagtail,chimeno/wagtail,KimGlazebrook/wagtail-experiment,100Shapes/wagtail,zerolab/wagtail,davecranwell/wagtail,gasman/wagtail,willcodefortea/wagtail,Toshakins/wagtail,dresiu/wagtail,JoshBarr/wagtail,mayapurmedia/wagtail,thenewguy/wagtail,kurtw/wagtail,iansprice/wagtail,darith27/wagtail,darith27/wagtail,nimasmi/wagtail,torchbox/wagtail,Klaudit/wagtail,tangentlabs/wagtail,janusnic/wagtail,jnns/wagtail,gogobook/wagtail,KimGlazebrook/wagtail-experiment,tangentlabs/wagtail,lojack/wagtail,serzans/wagtail,chimeno/wagtail,serzans/wagtail,wagtail/wagtail,nutztherookie/wagtail,taedori81/wagtail,rsalmaso/wagtail,gasman/wagtail,rv816/wagtail,Klaudit/wagtail,janusnic/wagtail,WQuanfeng/wagtail,nrsimha/wagtail,benemery/wagtail,bjesus/wagtail,davecranwell/wagtail,iho/wagtail,iansprice/wagtail,KimGlazebrook/wagtail-experiment,kurtrwall/wagtail,mephizzle/wagtail,quru/wagtail,willcodefortea/wagtail,stevenewey/wagtail,dresiu/wagtail,zerolab/wagtail