Clean up Light Groups

[OttoWinter]

Description:

• In the docs and some of the code, we refer to "Light Groups"; the "Group Light" name is from the old GroupedLight implementation and makes less sense.
• Move getting the initial state from async_add_devices to async_added_to_hass (Fix flaky light group test #12843 (comment)); The former does the async_update before we register our callback. Theoretically it could happen that a state is published between when add_deviceswhen our subscription is established. Then we would miss a state.
• Remove light groups from .coveragerc. I didn't know what that file did when doing the initial PR.

Related PR (if applicable): #12229

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#4739

Example entry for configuration.yaml (if applicable):

light:
  - platform: group
    name: Kitchen Lights
    entities:
      - light.kitchen_ceiling_lights
      - light.kitchen_under_cabinet_lights
      - light.kitchen_spot_lights
      - light.pendant_lights

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.

[balloob]

You should just call await self.async_update() here, updating state is done right after the call to async_added_to_hass: https://github.com/home-assistant/home-assistant/blob/dev/homeassistant/helpers/entity_platform.py#L257-L260

[OttoWinter]

Good catch, thanks.

[OttoWinter]

Putting this in the 0.65 milestone because the merged docs have the updated default name "Light Group".