Automatic checksum generation on maven UI upload

in nexus 2 uploading an artefact used to generate sha1 and md5 hash files for both the uploaded jar and the generated pom.

In nexus 3 these files are not created.

For compatibility reasons I would prefer to either:

• generate checksum files automatically
• make checksum file generation optional
• Hint that checksum files are not created 'automagically'

Rational:

• Artefacts without valid checksum files will generate nasty warnings in build systems like maven or gradle