#3506108: Editing code components as overrides, step 1

1. Overrides are represented by JavaScriptComponent config entities with a block_override property storing the base ID of the block plugin to override.
2. A new accordion group in the left sidebar lists these reusing the previously written CodeComponentList component, and with the help of useGetCodeComponentsQuery which can now return overrides vs. non-overrides.
3. Three default config entities are shipped. Page title, which would have been the fourth one, had to be deleted, because we can't render the page title block in XB.
4. Overrides can't be added to components, and they can't be renamed or deleted.
5. Instead of allowing users to define props, we present example data.
6. Preview inside the code editor is generated using the example data. New utility functions are introduced to help with that, and there is now unit test for the most complex one.