Organize our polyfills and standardize on core-js

What does this MR do?

• replaces all of our existing ES6 polyfills with their core-js implementations.
• migrates all of our existing browser polyfills from extensions/foo.js to commons/polyfills/foo.js
• moves jQuery extensions into commons/bootstrap.js

Are there points in the code the reviewer needs to double check?

all tests should pass

Why was this MR needed?

we didn't have one standard place to include and reference all of our ES6/Browser polyfills.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?