Rework & fix artist nb tracks/albums counting

This set reworks the way we count artists' number of tracks & albums, by relying on database triggers rather than manual updates. Before this, we would never decrement the number of tracks when a media was externally deleted, which at best would cause incorrect information being displayed, and which could also lead to database checks failing, such as the one in #126 (closed)

Edited by Hugo Beauzée-Luyssen

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