Add language filter to sync-ingredients management command.

[navyjosh]

Proposed Changes

• Allow user to specify a language when syncing ingredients
• example: python manage.py sync-ingredients --languages=en,fr,es

Please check that the PR fulfills these requirements

• [x] Tests for the changes have been added (for bug fixes / features)
  • wger.nutrition.tests.test_xync.TestSyncMethods has been slightly modified to ensure uri was called with proper filter.
• [x] Added yourself to AUTHORS.rst

Other questions

• Do users need to run some commmands in their local instances due to this PR (e.g. database migration)? No