Add artwork information to releases and release-groups in json-ld data.
Add instrument to get_plural_type
Remove unncessary workaround from INSTALL.md

INSTALL.md contained a workaround for an issue that had already been
fixed years ago, in commit da4ea6ea313a558ccaaa16e851401e56b5cb245a
(cf. MBS-4103).
Guess case: remove a bunch of unnecessary files
Merge remote-tracking branch 'chirlu/instrument_plural' into beta

* chirlu/instrument_plural:
  Add instrument to get_plural_type
Merge remote-tracking branch 'chirlu/install' into beta

* chirlu/install:
  Remove unncessary workaround from INSTALL.md
Merge remote-tracking branch 'bitmap/gc-bleh' into beta

* bitmap/gc-bleh:
  Guess case: remove a bunch of unnecessary files
Merge branch 'json-ld-artwork' into beta

* json-ld-artwork:
  Add artwork information to releases and release-groups in json-ld data.
Update pot files using current code and production database.
Update translations from transifex.