Add back to top web component from toolkit

[nhjohnstone]

[github-actions]

Branch nhjohnstone/issue-910-add-back-to-top-web-component-from-toolkit created!

[nhjohnstone]

scrolling was not working correctly after updating to bootstrap 5

fix is to add this to _reboot.scss for il-fw-theme
overrides line 35

:root {
  scroll-behavior: auto !important;
}

also need to change the z-index of <il-back-to-top> to 100 so its on top of paragraphs

see the solution here and here

[trubach]

With this targeting the 5.x branch, it will need to be refactored

[nhjohnstone]

@trubach this have been included in html.html.twig in 5.x branch

[trubach]

Closing since this is done in the 5.x branch