v2.0 - New looks for text-generation-webui!

Changes

Improved the UI by pushing Gradio to its limits and making it look like ChatGPT, specifically the early 2023 ChatGPT look (which I think looked better than the current darker theme).
- I have used chatbot-ui (the "legacy" version, v1.0, April/2023) as a reference for the old ChatGPT styles, and copied a lot of CSS and some icons from there. Credits to chatbot-ui!
- Mobile support is now much better, with collapsible sidebars added for easier navigation.
- Better, more readable fonts in instruct mode.
- Improved "past chats" menu, now in its own sidebar visually separated from the chat area.
- Converted the top navigation bar (Chat / Default / Notebook, etc.) into a vertical sidebar on the left.
- Reduced margins and removed borders throughout the UI. The "Parameters" tab looks much tidier now, closer to how Gradio is used in AUTOMATIC1111/stable-diffusion-webui.
- Updated Gradio from version 4.26.0 to 4.37.1, bringing important security fixes.
- For people who feel nostalgic about the old colors, a new --old-colors flag has been added to make the UI as similar as possible to its previous look.
Improved HTML rendering for lists with sub-lists (sub-items were not previously rendered correctly).
Allow more granular KV cache settings (#6561). Thanks @dinerburger.

openai extension fix: Handle Multiple Content Items in Messages (#6528). Thanks @hronoas.
Filter whitespaces in downloader fields in model tab (#6518). Thanks @mefich.
Fix an issue caused during the installation of tts (#6496). Thanks @Aluisio-Pires.
Fix the history upload event in the UI.