No Data Import Handler UI in Solr 9

Data Import Handler Back-End was moved to https://github.com/rohitbemax/dataimporthandler

However, DIH UI was simply removed from Solr 9 and not moved to this repository.

Re-adding DIH UI requires changes in some common files, like app.js and service.js.

For that reason, DIH UI should be a part of the main Solr distribution, not provided externally.

Please bring Data Import Handler UI back to Solr.

See also: https://github.com/rohitbemax/dataimporthandler/issues/32