All-document-mappings file now excludes non-valid "open" attachments

A change to the redirector-generation script to remove the enforced precedence of 301 over 410 exposed the inclusion of now-invalid, but previously imported attachments in the all document mappings file.    https://www.pivotaltracker.com/story/show/55377246
Merge pull request #776 from rjc123/patch-2

https://www.pivotaltracker.com/story/show/55377246

All-document-mappings file now excludes non-valid "open" attachments