Long month names in locale.datepicker.js not using context properly

As found by @droplet in #1452188: New UI for string translation, long month names in locale.datepicker.js in Drupal 8 do not use context for long month names, and therefore the long month names get recorded for translation (and used translated) without their proper context. Drupal 8 did add context support for JS translation that was missing in Drupal 7 (in #488496: Implement missing msgctx context in JS translation for feature parity with t()), so it should not be possible to add the 'Long month name' context to the JS strings in locale.datepicker.js now and close this bug.