Add the thumb file to missing and invalid files work

[TheElo]

When hydrus throws out invalid files it moves them to a folder, a text file with tags, and notes the missing hash. The user, if unlucky, may end up with files with no or limited tags and no URLs. A thumb file which already exists, should also be moved instead of just deleted, to this folder. The user then can use reverse image search to find the file or at least get an idea what even was lost. The current automated process removes the thumb file without even asking, removing that option from the user.