Unify logic for setting metadata via files and via the api/process

Closes #694 (closed). With these changes, MetadataEditRequestHandler handles both metadata files and requests to edit metadata through the api/by calling Upload.edit_upload_metadata, and we thus have the same validation and json format in both cases. Setting metadata via the ArchiveParser is also disallowed, at least for now (if this should be allowed, we should probably use the same class also in this case). Also refactored MetadataEditRequestHandler a bit to make it use a simpler and more consistent terminology.