Docs housecleaning, etc.

[shawnbot]

Documentation updates:

• Fix a bunch of React rendering warnings around missing key in iterators, etc.
• Add Box examples
• Beef up Link examples
• Tidy up Flash and Link component JS and examples
• Standardize example subheadings with ExampleHeading
• Remove unused Primer CSS imports (e.g. primer-support doesn't even have CSS!)
• Remove unused ThemeProvider from examples Page component
• Move CSS, Meta, and Page components into examples/ (note: this causes x0 to build static HTML for each of these, which is probably not what we want!)
• Use theme fonts and lineHeight values in the (docs) CSS component, rather than hard-coding them

Component updates

• Move style props management (for passing props through to style={{...}}) to stylizer() in props.js, and use that in Block. Ref: Add Caret component #58 (comment)

Dependency updates

• Pin x0@5.0.4 to fix HMR issues
• Pin styled-components@3.3.2 (it's still necessary as a dev dependency because I guess x0 or kit use it?)

Etc.

• Ensure that classifier() doesn't duplicate class names, which should make testing easier.

[shawnbot]

One thing you'll see here is that moving the CSS, Meta, and Page components into examples/ causes static pages for them to be generated by x0. I guess that's how x0 works (edit as of version 5, it is!), so maybe the fix for #59 is to move each component <Example> into a separate file? Something to look at next week.