Entity Registry: disable entities

[balloob]

Description:

Add ability to disable entities from the entity registry. When a disabled entity is passed to async_add_entities, it will be ignored.

To disable an entity, add the key disabled_by: user to an entity. We might in the future also get disabled_by: hass if an entity is doing I/O in the event loop or misbehaving in any other way 👍

For integrations to make sure that they don't do any extra work, they should make sure to only listen for updates to their entities when async_added_to_hass is called.

Checklist:

• The code change is tested and works locally.

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.