Consolidate button.css into style.css

[jotaen4tinypilot]

style.css is already importing button.css, so we automatically have both when we import style.css in web components.

However, in almost all of our web components, we still import both (e.g., see the <update-dialog> as an example.

I’d argue that the button style is as fundamental as any other style definition in style.css, so we could consider refactoring this. E.g.:

• We copy over the contents from button.css to style.css, and remove all @import "css/button.css directives.
• We eliminate all redundant @import "css/button.css, but keep the button.css file as separate file for tidiness.
• We drop the button.css import from style.css, but then we’d be forced to import it in all places where we use it.