Backport Versions menu to 13.12

What does this MR do and why

Backports the Versions menu to 13.12 (#1238 (closed)).

In addition to cherry-picking commits from the Versions menu MR over to this branch, I needed to make a few additional changes for this to work on the older site:

• Patch GitLab UI (this bug comes up in docs 13.12 when adding GlDropdown; interestingly it is not present in docs 14.10 or 15.x)
• Backport SVG updates in order for GlUI icons to appear
• Adjust styles/classes since we were using Bootstrap instead of GitLab UI CSS in 13.12
• Add the regenerator-runtime babel polyfill in order to support use of async/await (https://stackoverflow.com/a/60823029/1940172)

How to test

1. View the site in the Review App: http://667-versions-menu-1312.35.193.151.162.nip.io/index.html
2. Verify the Versions menu shows the same Versions as the main branch, docs.gitlab.com
3. Verify the menu displays correctly at large and small widths
4. Check for errors in the browser console

Caveats for the Review app:

• The menu will default to 15.2 in the Review app because the Review app does not deploy older versions with the same URLs as production (e.g, docs.gitlab.com/15.1/whatever)
• Links to other versions will not work in the Review app