Introduce styles for disabled buttons with `primary`, `secondary` and `danger` variants.
Also added `:hover` background color change for `secondary` and `danger` variants, because it was missing.
Following styles in Desktop App and Mobile Figma design:
Disabled Primary button - https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1077%3A69208
Secondary buttons - https://www.figma.com/file/2H0BQsi8MgC5MpjNtViJxB/Comm-%2F-Mobile-Design-System?node-id=480%3A4797
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