TranslationProject: Fix potential 500 when last_submission.unit is None
Cleanup: Remove unused parameter
Don't pass the goal slug if we don't have it
Cleanup: Avoid hitting DB unless necessary
Bugfix: Provide number of editor rows for AnonymousUser

Before this the template was setting no value for chunkSize when an
anonymous user visited the editor, and therefore they were experiencing
a JavaScript failure resulting in editor not being rendered.
Bugfix: Get the default rows count for AnonymousUser when getting units

This allows populating the editor for anonymous visitors.
Bugfix: Return empty alternative languages queryset for AnonymousUser

Otherwise the current unit is not displayed for anonymous visitors.