Abstract reviewing: show warning when reviewer roles not applied (no tracks)

[pferreir]

When there are no tracks in an event, only judgment is possible (no reviewing). That means any reviewing roles set using the "Roles" dialog will be ignored.
The idea would be to add a warning in such case. Something like: "Since this event has no tracks, reviewer roles are disabled. Only judgment allowed."

[bpedersen2]

This actually also applies to abstracts without a track set. These are currently not reviewable.

Another solution would be to allow the "all tracks" reviewers to review also proposals without track set (but this seems tricky with the current db layout).