Resolve #3583684 "Symmetric content inputs"

Closes #3583684 (closed)

The aim here is to achieve the equivalent functionality for component trees stored in content entities as !898 (merged) does for component trees stored in config entities.

tmgmt_content will be need to do the actual translation as content_translation relies on having a field which we don't have for component trees. The tmgmt integration will be done in the follow-up #3590948 (closed)