Allow to bulk create / update corresponding entity references for existing entities

Problem/Motivation

When installing the module on an existing site, there's no way to bulk create / update corresponding references for the existing entities and their fields referenced in the CER entities.

This is even more problematic because of 🐛 Update existing references on save Needs work

Steps to reproduce

Proposed resolution

• Similar to what for example pathauto offers, add a "Bulk update" page in the administration area.
• Allow selecting the CER entities to bulk update their referenced entities.
• Allow selecting if only new references should be created or also existing should be updated.
• Optionally, add a views bulk operation for entities "Bulk update corresponding entity references"
• Optionally, add a short link on the CER list to "Update Entities", which preselects the entity on the bulk update form.

Remaining tasks

User interface changes

API changes

Data model changes