Implement secure cloud sandboxes with E2B

[jamesmurdza]

Description

This PR removes the built in node-pty terminal and adds integration with E2B's sandbox API. This fixes security issues with deployment and allows any number of sandboxes to run simultaneously.

It also fixes several bugs:

• Fixes a bug where new socket connections are made every time React refreshes the virtual DOM.
• Fixes a bug limiting the total number of user sandboxes to eight.
• Adds error handling to socket event listeners, so file or execution errors don't crash the whole server.

Implementation

Notes/limitations:

• The preview panel now dynamically updates it's URL to the correct sandbox, but the port number is still hardcoded.
• Sandboxes are closed immediately when the last browser tab using the sandbox is closed.
• As before, files updated in the editor update in the terminal, but not the other way around.

Terminology: Because of the word "sandbox" is already commonplace in the code, I decided to call E2B sandboxes containers in the source. However it would make a lot of sense to call those sandboxes and call sandbox projects projects.

Testing

To test this PR:

1. Add E2B_API_KEY to /backend/server/.env (available for free at E2B).
2. Run the frontend and backend, and open any sandbox.
3. Commands run in the terminal will indicate a fresh Debian Linux system.
4. Run npm install && npm run dev in the terminal and refresh the preview panel to see the live preview.
5. Edit a source code file and save changes to test hot reloading.

[ishaan1013]

@jamesmurdza lgtm, great work