fix(Modal): Lock focus

[cehsu]

These changes:

• Lock focus in modals (via react-focus-lock)
• Adds an example with a dropdown (to illustrate how returnFocus is handled)

Next steps:

• add VRT with puppeteer, and tie into CI
• consider using react-focus-on for modals as recommended by react-focus-lock
• close on escape keypress (Firefox)

[codecov]

Codecov Report

Merging #41 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #41   +/-   ##
=======================================
  Coverage   68.36%   68.36%           
=======================================
  Files          22       22           
  Lines         433      433           
  Branches       91       91           
=======================================
  Hits          296      296           
  Misses        107      107           
  Partials       30       30

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[kylealwyn]

This looks lovely!

Also, not sure if introduced here (probably not) but noticed in the long content example that the top of the modal is cut off and can't scroll up to see it.

[cehsu]

@kylealwyn the scrolling issue is indeed new, good catch, investigating fixed.

[kylealwyn]

🎉