Split component - third pass

[stephaniehobson]

• RTL support
• fallback support for non-grid browsers
• finalize class names Block component class names #356
• increase padding between sides at larger breakpoints
• full support for .l-reverse
• change is-inverse to l-reverse
• mobile support for flush and pop classes
• class to hide the image on mobile only
• overflow bug
• cross browser testing
• documentation

Description

Adds a new component to replace hero, feature, and billboard.

• I have documented this change in the design system.
• I have recorded this change in CHANGELOG.md.

Issue

Fix #326

Testing

• LTR
• backwards compatability

[stephaniehobson]

Note to self: see if this component can handle the trailhead designs on /accounts and /firefox/new

[stephaniehobson]

Also, it's on demo if you want to see it in action:
https://demo1--mozilla-protocol.netlify.com/demos/block.html
https://demo1--mozilla-protocol.netlify.com/demos/block-outlines.html

[stephaniehobson]

I expected us to have overflow-x:hidden on body but we don't. I assume it's for a good reason.
That means this component needs to safe guard against a crawl bar. I think that means it can't be nested in mzp-l-content.

[craigcook]

This is back up on demo1 for the moment, but one of the links changed so re-commenting here.

Regular examples: https://demo1--mozilla-protocol.netlify.app/demos/block.html
Fancy interactive demo: https://demo1--mozilla-protocol.netlify.app/demos/block-demo.html