Display player errors and prepare for client websockets

[Neraste]

This PR prepares for the use of websockets on the client and has numerous side effects:

• Rename playlist date_played as date_play;
• Playlist is integrated to the digest view;
• Playlist entries in digest view have minimal information to reduce request size;
• Playlist entries in digest have no pre-calculated date of play anymore, this has to be computed on the client now;
• Played and queuing playlist entries can be accessed in a library fashion (using pagination), their route was updated to use the played/queuing terminology;
• Player errors can be fully accessed (initial aim of the PR).

With this design, the digest route can be called to initialize the client to the latest state of the server. When implemented, websockets on the client will ensure punctual updates of the client. For the time being, the digest route can be polled.

[codecov]

Codecov Report

Attention: Patch coverage is 98.57143% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.65%. Comparing base (7b916ae) to head (29c40cd).

Files	Patch %	Lines
dakara_server/playlist/serializers.py	97.14%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #155      +/-   ##
===========================================
- Coverage    98.69%   98.65%   -0.05%     
===========================================
  Files           73       74       +1     
  Lines         1836     1852      +16     
===========================================
+ Hits          1812     1827      +15     
- Misses          24       25       +1     

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