Handle common issues on import

When a coordinator imports files, Montage should:

[ ] warn about deleted files #30
[ ] resolve moved files #97
[ ] resolve full Wikimedia Commons URLs
[ ] resolve URL encoding issues
[ ] resolve common typos (spaces or underscores at the beginning or end of the filename)
[ ] warn users when files are not imported #108
[ ] save a log of files that were not imported #108
[ ] provide an easy tool to figure out which files from your source list are not in the round #140
[ ] ensure there are no duplicate entry errors #146

hatnote / montage