Fix theme urls

[oskarrough]

We can't load themes via './themes/x.css' when r4-app is loaded from a CDN script.

This adds a new cdn attribute on <r4-app>.

• Set it to true to load themes from jsdelivr cdn
• Set it to a url where components are built, like above. Allows to control exactly which version to load

On https://github.com/radio4000/radio4000.github.io/blob/main/index.html we should then be able to just set cdn on r4-app and it'll load latest from jsdelivr.

Closes https://github.com/radio4000/components/issues/140

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