Change default UX light color temperature units to Kelvin

[GitHubGoody]

I just created my first scene in HA after using ST for years. I went to set the color temperature of some lights and I thought I messed something up since the values were in the 100's vs. 1000's. Using Kelvin in the UI seems like a no-brainer if we want to lower barriers to entry and welcome less technical users to the platform. This is for two reasons:

1. Most people have never heard of a mired, but they have read Kelvin ratings on the side of light bulb boxes and in other smart light apps.
2. The spectrums are reversed (a low mired value is more yellow instead of blue like Kelvin, and vice versa), which is counterintuitive.

After giving this (https://visualsproducer.wordpress.com/2020/11/29/mireds-versus-degrees-kelvin-for-colour-temperature/#:~:text=The%20formula%20to%20go%20from,2700K%20equals%20370%20mired%2C%20etc.) a quick read it seems the simple implementation would be to change the UI frontend to a Kelvin scale and then set the mired on the backend to 1,000,000/the UI K (e.g. 3000K = 333mired).

Here's the backend discussion about mireds vs. Kelvin: home-assistant/architecture#564

While waiting for the architecture to catch up, it seems the front end could be modified to adjust at the UI (1,000,000/mired) or (1,000,000/K) depending on which way you're going. In other words, if all the integrations are currently built using mireds, don't change anything on the backend...just change the frontend to accept K from the user and output mired to the backend (and vice versa for changes made external to that session).