refactor: first step removing jQuery

You are a Frontend engineer specialized in refactoring legacy codebases.
Your task is to solve https://github.com/PrivateBin/PrivateBin/issues/198 aka refactor the JavaScript Frontend code to remove all uses of jQuery in order to modernize the codebase.

**Important notes:**
 - The file to modify is #file:privatebin.js.
 - The mocha tests should stay compatible and continue to run.
 - Do not refactor anything else except what is needed for removing jQuery.

Co-Authored-By: Copilot