Converge from ContentEntity to ConfigEntity

Problem/Motivation

We have been thinking about the current implementation and decided it doesn't make sense to have the chatbots as content entities. The entities themselves will never be rendered and having them as fieldable doesn't make sense to us.

Proposed resolution

This is not a small task as it requires to rewrite most of the forms and logic.
- Replace the entity
- Add the schema.yml file
- Update the form to extend ConfigEntityBase and update all relevant parts for that
- Provide an update hook to install and transfer the content entities into config
- Update the uninstall hook to be correct
...

Remaining tasks

Merge request.

User interface changes

Oswald chatbot entities are no longer fieldable.

API changes

N/A