Merge branch 'master' of https://github.com/puckowski/Sling.js

Merge branch 'master' of https://github.com/puckowski/Sling.js

Update Sling.js to version 21.1.0

* Update Sling.js to version 21.1.0.
* Fixes for lifecycle hook calls on recycled nodes.

Update README.md weights

* Update README.md weights for Sling core.

Try to improve automated test stability

* Try to improve automated test stability. Run two tests later to try to
  avoid animation end change detection calls which were causing conflicts.

Bug fix Sling Reactive clearSubscription

* Bug fix Sling Reactive clearSubscription.
* Bump version to 20.3.1.
* Add automated test for Sling Reactive bringing the total to 198.

Add test for new terser markup function

* Add automated test for new terser markup function which brings the
  total test count to 197.

Update package.json

* Update package.json.

Update package.json

* Update package.json.

Revert visibility change

* Revert visibility change as we are using display: none instead and do
  not want element to impact page layout using component scoped CSS has
  been fully applied.