Follow-up of request #12733 and request #18451

The announcement for the end of support of Internet Explorer 11 has been made a while ago. On Tuleap 12.7, Tuleap will not function at all on Internet Explorer 11. There will be "script errors" because Tuleap will ship ES2015+ code, that Internet Explorer 11 does not understand.

This request will deal with listing and cleaning up all the special adjustments that have been made to support Internet Explorer 11 over the years.

1. Change the target browsers in Babel configuration to stop transpiling for IE11. This will drop most of the Babel polyfills.
2. drop the DOM4 polyfill
3. drop the Fetch polyfill
4. drop the Regenerator polyfill (async / generators)
5. drop the CSS.escape polyfill
6. drop the polyfills in TLP documentation
7. drop the Custom Elements polyfills
8. drop the Intl polyfills (@formatjs/stuff)
9. drop the polyfillCustomEventIE11 in help dropdown (FlamingParrot)
10. drop .woff fonts (in favor of .woff2)
11. stop transpiling some node_modules to ES5
12. drop the navigator.msSaveBlob() for Cross Tracker Search widget CSV export
13. drop the "Esc" key matchers from TLP (all others report "Escape")
14. use https://developer.mozilla.org/en-US/docs/Web/API/Clipboard/writeText and drop the IE11-only API to copy to clipboard

Later, we will also be able to introduce "new" technologies that were not possible to use before due to the IE11 support, such as:

• use https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams and other new Browser APIs
• use "once" modifiers for event listeners https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#Parameters
• use CSS grid
• try using <dialog> tags instead of custom-building TLP modals with javascript (but check accessibility)
• try using <summary> and <details> tags instead of javascript collapse/expand (but is has accessibility issues)
• upgrade CKEditor to v5+
• use web-streams for the Artifact Modal File upload (instead of NodeJS readable-streams)
• use type="module" for <script> tags
• use luxon instead of moment + moment-timezone (moment-timezone is ~700kB)
• use CSS Custom Properties instead of building 6 or 12 CSS stylesheets per plugin / screen for themes
• use Custom Elements with Shadow DOM (aka Web components)