Add more information about opponents source

[Drarig29]

Currently, there is only match.opponent[1|2].position, which gives the number of the source match. But if we want to know the source match, we have to do computations.

Doing as Toornament in https://developer.toornament.com/v2/doc/organizer_bracket_nodes could be a better approach.

We would replace replace position by source_type (none, winner, loser) and source_match_id.

This would simplify the viewer too (about origin hint).