Merge the IIIF and file imports

After backend#1298 (closed) and tasks#33 (closed), DataImportMode.IIIF gets merged into DataImportMode.Files. A file import now allows all currently supported DataFile formats at once.

The enum in js/config.js and the checks in vue/Import/Files/ImportFromFiles should be removed. The backend will be solely responsible for checking the MIME types, so that we do not have to update the frontend each time we add support for a new file type. The frontend will have to display all errors properly.

A mode should no longer be sent to CreateDataImportFromFiles, since it will always be set to Files by the backend.