Remove ViewUiConverter

Problem/Motivation

Found in #2512718: EntityManager::getTranslationFromContext() should add the content language cache context to the entity → .

ViewsUiConverter upcasts a config entity based on the current session.

While this only happens on views admin pages, it's a bad pattern to randomly change config or content entities in paramconverters.

Proposed resolution

Do this somewhere else, probably in the controller(s).

Remaining tasks

Fix the bugs
Remove the proxy class
Celebrate

User interface changes

API changes

Data model changes