Delete operation is not restricted while site is in read only mode

Problem/Motivation

When site is in read only mode no entities can be created or updated. But it is still possible to delete entities from web UI. I believe read only mode should also restrict "Delete" operation.

Steps to reproduce

1. Switch on "Read only" mode here '/admin/config/development/maintenance'.
2. Try to create a node - observe that validation fails and it is not possible.
3. Try to update a node - observe that validation fails and it is not possible.
4. Try to delete a node - observe the successful delete operation.

Proposed resolution

The "Delete" operation should be not possible while Read Only mode is on. Maybe it should be optional with configuration setting to force it.

Remaining tasks

Create MR

User interface changes

API changes

Data model changes