Remove jQuery dependency

The jQuery library is used to add functionality to vanilla JavaScript. It was once very useful for cross-browser compatibility, and it abstracted a number of things that were difficult in regular JavaScript.

This is no longer the case. We should drop jQuery from our codebase in order to modernize our frontend code and drop a large dependency that we load on every page of the site.

jQuery is in use in a few places:

• badges.js: Removed in #1690 (closed).
• toggle_popover.js: Removed in !4322 (merged)
• dom.js: Removed jQuery code in !4111 (merged).
• clipboard.js
• Bootstrap 4.x. See #1684 (closed).