Unpublished translations should fallback to the original language

Problem/Motivation

From #2951294.9:

1. When we have a default revision having only a published English translation, both /node/1 and /it/node/1 return a 200.
2. When we have a default revision having a published English translation and an unpublished Italian translation, /node/1 returns a 200 and /it/node/1 returns a 403.

If I understand correctly, what @Berdir is suggesting would make case 2 behave as case 1: CT's language fallback would kick in and the English translation would be displayed also on /it/node/1.

Proposed resolution

The logic from content_translation_language_fallback_candidates_entity_view_alter should apply to entity_upcast context as well.

Remaining tasks

User interface changes

API changes

Data model changes