Improve management of instrumental files

[Neraste]

The has_instrumental field has been replaced by the instrumental_file and instrumental_track fields.

Related to:

• Improve management of instrumental files dakara-feeder#47
• Improve management of instrumental files dakara-player#143

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.70%. Comparing base (5a30ab6) to head (9d28ec7).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #190   +/-   ##
========================================
  Coverage    98.69%   98.70%           
========================================
  Files           83       84    +1     
  Lines         1994     2006   +12     
========================================
+ Hits          1968     1980   +12     
  Misses          26       26           

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