Roles: allow importing from registration lists

Currently, role members have to be added manually. Please allow importing role members in e.g. CSV format, and/or from registration lists.

No files, tests, or entry points are named. Start by locating the role-member management and registration-list entry points, then determine how CSV or registration-list imports should fit together. Done means role members can be imported through the agreed format or source instead of being added only manually.

Written by the indexing model from the issue text.