Add the link template "add-form" to content entity types

Problem/Motivation

The "add-form" link template has been introduced in #2666792: Provide a route provider for add-page of entities, but it has not been set on all entity types. I've ran into a situation, where I need this link template for the node entity type.

Proposed resolution

Add the add-form link template to all content entity types.

Remaining tasks

User interface changes

API changes

Data model changes