Major improvements: new functionalities, better code readability, fixed bugs, improved interface, state-of-the-art free open-source model as default

[costabm]

Changes and improvements:

• The app now has a "Forget conversation" button. This allows cleaning the memory of the chat, without erasing the memory of the PDFs.
• The default model is now one of the latest free open-source models.
• Replaced PyPDF2.PdfReader with pypdfium2.pdfium (higher performance stated here: https://github.com/py-pdf/benchmarks credit is given to another pull request by: yiwei-ang:feature/pdfium)
• A message is displayed when the documents have been processed.
• The code has been re-structured for better readability. A few explanation comments were added.
• Bug fixed where the user message remained visible after submission and was re-sent automatically when the page was refreshed.
• New human icon (previous one was not available).
• The relevant packages have been updated and compatibility is assured.