Participant roles: filter view by (un)registered status

[pferreir]

Right now there is no way to get a list, for instance, of all authors who have not registered yet.

An idea would be to add an "Unregistered users" option in here:

[pferreir]

As @ThiefMaster has already pointed out on Mattermost, we have already got this option in the "Contribution List".

We should discuss whether the option should be moved to the "participant roles" page or just kept in both places.

[ThiefMaster]

Having it in both places probably makes sense - no need to take it away elsewhere IMO.