feat: allow handler to redact handler attachments

[aatkin]

relates #3190

Allow handler to redact also other handler attachments. Refactored model/allowed-redact-roles and model/apply-attachment-permissions, and amended api test with test case where handler redacts another handlers attachment.

Other changes

• handling users may now see which attachments were redacted in event/redacted-attachments

Checklist for author

Remove items that aren't applicable, check items that are done.

Reviewability

• Link to issue
• Consider adding screenshots for ease of review

Backwards compatibility

• Feature appears correctly in PDF print

Documentation

• Update changelog if necessary

Testing

• Complex logic is unit tested
• Valuable features are integration / browser / acceptance tested automatically

Follow-up

• New tasks are created for pending or remaining tasks

Screenshots

Handler

Applicant

[Macroz]

This is nice naming 👍