Incorporate some of the nice things from MallocArray's configs

[geerlingguy]

I really liked a few things I saw in @MallocArray's ESPHome configuration for the ONE, and may incorporate them here:

• Adding a switch for Fahrenheit / Celsius, so you can easily swap between them.
• Using substitutions at the top of the file instead of secrets, which allows all settings to be self-contained in the one file. (There are some advantages to a secrets file though...).

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