SEARCH-383: JSON return an array called 'artist' instead of 'artists'

This commit applies

* https://bitbucket.org/metabrainz/search-server/commits/f1dce7d78cb2dd74be34b649492cb02021b0d39d
* https://bitbucket.org/metabrainz/search-server/commits/02de43c44757ee15a24ce7478a30fdd950b29be6
SEARCH-379: JSON recording response embeds track artist credit inside an artist credit

This commit applies

* https://bitbucket.org/metabrainz/search-server/commits/fd4a3cab0909272041956ab38d8734300ebde6d6