assistant2: Render saved conversations inline instead of in a modal (#11630)

This PR reworks how saved conversations are rendered in the new
assistant panel.

Instead of rendering them in a modal we now display them in the panel
itself:

<img width="402" alt="Screenshot 2024-05-09 at 6 18 40 PM"
src="https://github.com/zed-industries/zed/assets/1486634/82decc04-cb31-4d83-a942-7e8426e02679">

Release Notes:

- N/A