Storybook improvements: CSF 3 and visual tests

[Andrey Morozov]

• Rewrite stories in a simpler format, CSF 3. See Button or Label stories as an example. Each story must cover specific scenario or prop, and be as simple as possible. Showcase and ShowcaseItem utility components could help organize story's layout.
• For majority of stories there should be a dedicated visual test in addition to other corner-cases.

List of components:

• #1613
• #1610
• #1603
• #1596