Add modal component

[dpikt]

Adds a modal component that is untethered to the usage of redux / the HOC pattern. We've been using this along with local state on Prosci and it's been simple and intuitive so far (far less magical than the old method).

Ideally, I'd like to include this pattern rather than the HOC one as the recommended approach in the next version of the onboarding project (despite there being fewer examples in our projects). Let me know what you think about that idea!

Note: ideally modal from lp-hoc would render this component directly, however that's not possible at the moment since it would cause a circular dependency between the libs. This could be addressed later if it becomes an issue with making updates to one or both.

Author Checklist

• [x] Add unit test(s)
• [ ] Update version in package.json
• [x] Update documentation (if necessary)
• [x] Add story to storybook (if necessary)
• [x] Assign dev reviewer

[dpikt]

@chawes13 what do you think about including this approach in the onboarding project?

[chawes13]

@dpikt love it

[dpikt]

Resolved by #443