refactoring from jQuery to modern JavaScript

[bjohansebas]

As discussed in #1568, these will be the next steps to stop using jQuery in favor of the browser's API:

• First PR (Refactor Menu): new design and logic for the menu #1590 #1568

  • Remove dropit
  • Remove functions related to dropit
  • Refactor the menu according to the new structure
  • Create styles according to the new structure
  • Add new menu functionality in a new file

• Second PR (Remove Unused Functions):

  • Remove ismobil.js
  • Remove logic for page editing (no longer used since the footer was updated)
  • Remove logic for window.location.pathname === '/3x/api.html' (no longer used since almost all URLs now go to /en/*)
  • Remove function related to #tags-side-menu

• Third PR (Adapt get elements):

  • Update scroll top logic
  • Adapt element retrieval and events
  • Add overlay logic to the menu
  • Adapt code highlighting

• Fourth PR (Adapt Functions):

  • Update logic for the i18n banner
  • Update logic for the closer title (used in the API)
  • Adapt cookies logic to ES6

• Fifth PR (Remove jQuery Dependency, Remove jQuery  #1569):

  • Remove link to jQuery

[bjohansebas]

The search bar in dark mode is broken in terms of design. @chrisdel101 Should I fix it in the first PR or better in a separate PR?