feat(Sticky): add active prop

[layershifter]

Ref: SO.

This PR:

• add active prop to Sticky
• refactors Sticky tests (use domEvent, described props, other cleanups)

[codecov-io]

Codecov Report

Merging #2053 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2053      +/-   ##
==========================================
+ Coverage   99.76%   99.76%   +<.01%     
==========================================
  Files         148      148              
  Lines        2561     2573      +12     
==========================================
+ Hits         2555     2567      +12     
  Misses          6        6

Impacted Files	Coverage Δ
src/modules/Sticky/Sticky.js	100% <100%> (ø)	⬆️

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[levithomason]

Awesome, changes LGTM.

[levithomason]

Released in [email protected].