Database checker and fixer

I'd like to have a tool to check for consistence of database. This tool should check if the files are accessible and offer for a fix (replace it with an empty file, remove the submission, etc), maybe other things too.

This is the problem I came across: I was doing a DumpExport to migrate the database. The DumpExport found some inconsistency in the database. More specifically, some files got deleted (I created another issue explaining this: #635). The thing is that this error only appeared when trying to read the file, which was two hours after starting the DumpExport. I managed to fix it altering the database manually, but it was not the only corrupt file. So to correct each file I have to run the DumpExport, wait two hours so I can see the problem, correct it, and run again. This is not convenient.

Start by reading the DumpExport workflow and issue #635 for the reported missing-file inconsistency. Determine where database records reference files and how DumpExport currently discovers failures. Done means a checker can identify inaccessible files before or independently of a long export and offer the proposed repair choices.