Add unique index to ImportedHitsFile.filename

Conceptually there is only one hits file at a given path; if the
content has changed, then the hash will also have changed and it
will be reimported. This enforces this at the database level.
Validate uniqueness of ImportedHitsFile.filename
Merge pull request #404 from alphagov/uniqueness_of_imported_hits_files

Ensure that ImportedHitsFiles are unique by filename