Add limitlessled night light effect

[amelchio]

Description:

Some (all?) LimitlessLED bulbs support a "night light" that is even dimmer than the lowest brightness setting. This PR adds a light effect to activate that mode.

It also adds the effect_list property so effects can be selected in the frontend.

Related issue (if applicable): fixes #

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.github.io#<home-assistant.github.io PR number goes here>

Example entry for configuration.yaml (if applicable):

  - service: light.turn_on
    entity_id: light.hall
    data:
      effect: 'night'

Checklist:

• The code change is tested and works locally.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully.
• New dependencies have been added to the REQUIREMENTS variable.
• New dependencies are only imported inside functions that use them.
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.