Refs #3929 Forum; added migration of survey attachments

git-svn-id: https://svn.eionet.europa.eu/repositories/Naaya/trunk/eggs/naaya.content.bfile@21191 a0171fae-0946-0410-966d-b1775066cd26
Refs #3929 Forum; added entry in changelog

git-svn-id: https://svn.eionet.europa.eu/repositories/Naaya/trunk/eggs/naaya.content.bfile@21192 a0171fae-0946-0410-966d-b1775066cd26
Refs #3929 Forum; avoid failure on rerunning migration on same site

git-svn-id: https://svn.eionet.europa.eu/repositories/Naaya/trunk/eggs/naaya.content.bfile@21194 a0171fae-0946-0410-966d-b1775066cd26
Refs #3929 Forum; delete extfiles by marking them with .undo on disk

git-svn-id: https://svn.eionet.europa.eu/repositories/Naaya/trunk/eggs/naaya.content.bfile@21195 a0171fae-0946-0410-966d-b1775066cd26