Add a ``old_url`` arg for posts in base Blog importer for creating redirects.
Add a ``old_url`` arg for posts in base Blog importer for creating redirects.

SoLoHiC/mezzanine,wbtuomela/mezzanine,industrydive/mezzanine,wrwrwr/mezzanine,jerivas/mezzanine,frankier/mezzanine,agepoly/mezzanine,stbarnabas/mezzanine,dekomote/mezzanine-modeltranslation-backport,scarcry/snm-mezzanine,damnfine/mezzanine,dustinrb/mezzanine,vladir/mezzanine,SoLoHiC/mezzanine,stephenmcd/mezzanine,spookylukey/mezzanine,molokov/mezzanine,biomassives/mezzanine,promil23/mezzanine,joshcartme/mezzanine,jerivas/mezzanine,vladir/mezzanine,molokov/mezzanine,theclanks/mezzanine,jjz/mezzanine,jerivas/mezzanine,emile2016/mezzanine,frankchin/mezzanine,readevalprint/mezzanine,biomassives/mezzanine,saintbird/mezzanine,fusionbox/mezzanine,Skytorn86/mezzanine,sjuxax/mezzanine,Kniyl/mezzanine,frankier/mezzanine,eino-makitalo/mezzanine,dekomote/mezzanine-modeltranslation-backport,industrydive/mezzanine,Kniyl/mezzanine,emile2016/mezzanine,joshcartme/mezzanine,theclanks/mezzanine,sjdines/mezzanine,ZeroXn/mezzanine,viaregio/mezzanine,AlexHill/mezzanine,ryneeverett/mezzanine,molokov/mezzanine,sjuxax/mezzanine,Cicero-Zhao/mezzanine,sjuxax/mezzanine,geodesign/mezzanine,vladir/mezzanine,dustinrb/mezzanine,fusionbox/mezzanine,dsanders11/mezzanine,batpad/mezzanine,AlexHill/mezzanine,tuxinhang1989/mezzanine,agepoly/mezzanine,guibernardino/mezzanine,wbtuomela/mezzanine,scarcry/snm-mezzanine,joshcartme/mezzanine,promil23/mezzanine,gradel/mezzanine,emile2016/mezzanine,scarcry/snm-mezzanine,spookylukey/mezzanine,webounty/mezzanine,dustinrb/mezzanine,PegasusWang/mezzanine,Cicero-Zhao/mezzanine,gbosh/mezzanine,Cajoline/mezzanine,Cajoline/mezzanine,stephenmcd/mezzanine,damnfine/mezzanine,damnfine/mezzanine,mush42/mezzanine,wbtuomela/mezzanine,wyzex/mezzanine,ryneeverett/mezzanine,viaregio/mezzanine,Cajoline/mezzanine,orlenko/sfpirg,orlenko/sfpirg,theclanks/mezzanine,eino-makitalo/mezzanine,gbosh/mezzanine,saintbird/mezzanine,geodesign/mezzanine,mush42/mezzanine,tuxinhang1989/mezzanine,batpad/mezzanine,SoLoHiC/mezzanine,nikolas/mezzanine,dovydas/mezzanine,readevalprint/mezzanine,cccs-web/mezzanine,adrian-the-git/mezzanine,ZeroXn/mezzanine,geodesign/mezzanine,promil23/mezzanine,guibernardino/mezzanine,readevalprint/mezzanine,douglaskastle/mezzanine,wrwrwr/mezzanine,biomassives/mezzanine,frankchin/mezzanine,PegasusWang/mezzanine,wyzex/mezzanine,stbarnabas/mezzanine,viaregio/mezzanine,nikolas/mezzanine,gbosh/mezzanine,Skytorn86/mezzanine,christianwgd/mezzanine,jjz/mezzanine,gradel/mezzanine,orlenko/plei,stephenmcd/mezzanine,gradel/mezzanine,spookylukey/mezzanine,dsanders11/mezzanine,douglaskastle/mezzanine,christianwgd/mezzanine,jjz/mezzanine,tuxinhang1989/mezzanine,eino-makitalo/mezzanine,adrian-the-git/mezzanine,christianwgd/mezzanine,webounty/mezzanine,cccs-web/mezzanine,adrian-the-git/mezzanine,frankier/mezzanine,dsanders11/mezzanine,mush42/mezzanine,Skytorn86/mezzanine,orlenko/plei,orlenko/plei,PegasusWang/mezzanine,industrydive/mezzanine,dekomote/mezzanine-modeltranslation-backport,ryneeverett/mezzanine,saintbird/mezzanine,nikolas/mezzanine,dovydas/mezzanine,agepoly/mezzanine,orlenko/sfpirg,sjdines/mezzanine,frankchin/mezzanine,ZeroXn/mezzanine,douglaskastle/mezzanine,wyzex/mezzanine,webounty/mezzanine,dovydas/mezzanine,Kniyl/mezzanine,sjdines/mezzanine