Fix Matomo attributes

[nicolaskempf57]

This PR changes the way we handle recommendations content in Matomo.

Before

Each recommendation type has a content name following the pattern {type} recommendations except external that uses dataset externals.
All recommendations have the same content piece reco because loop.idx is not set.

After

Each recommendation type has a content name following the pattern {type} recommendations including external ones.
All recommendations have a content piece corresponding to the individual piece of content shown :

• a dataset or reuse recommendation has its title as piece
• an external recommendation has its url as piece

This allows Matomo to provide metrics for a type of recommendation but also for each piece.

[maudetes]

Seems to make sense!