Replace Java-Express with Javalin

[4Benj]

Description

Wow! It's my first proper PR in like 2 or 3 months... whoops.

This PR removes java-express from the project, and has replaced it with Javalin, which java-express was based on but wasn't up-to-date and no longer maintained (plus a bit bloated, whoops).

Not all routes have been tested (just made sure that the server is in a usable state such as the ability to login). I did test the documentation route and discovered that with Javalin you need to set the Content Type for html documents so, some things may not work correctly. If you would like me to test every route to make sure they work before approving of this just let me know through discord and I will (most likely tomorrow).

THIS WILL BREAK ANY PLUGINS USING THE HTTP SERVER

Please carefully read the Contributing note and Code of conduct before making any pull requests.
And, Do not make a pull request to merge into stable unless it is a hotfix. Use the development branch instead.

Issues fixed by this PR

Type of changes

• Bug fix
• New feature
• Enhancement
• Documentation

Checklist:

• My code follows the style guidelines of this project
• My pull request is unique and no other pull requests have been opened for these changes
• I have read the Contributing note and Code of conduct
• I am responsible for any copyright issues with my code if it occurs in the future.