New api for editing metadata

Creates a new api for editing metadata. The idea is that all metadata should be edited using the same mechanism.

The go-to method for editing metadata is nomad.processing.MetadataEditRequestHandler.edit_metadata. It is possible to edit a single upload (including upload level attributes), or a selection of entries defined by a query. The method can also be run with a verify_only flag to test out the operation and get feedback on errors etc, without actually executing it. If verify_only is False and validation succeeds, the request is executed by starting a process for each affected upload.

For the REST api, two endpoints are created: entries/edit and uploads/{upload_id}/edit.

Remains to do: extend tests, remove old code, make it so that metadata fetched from nomad.json files use the same logic.