Updates the assistant with some style quality of life changes.

## Changes

Restyled the conversation list

<img width="646" alt="CleanShot 2023-07-10 at 10 25 23@2x"
src="https://github.com/zed-industries/zed/assets/1714999/5c9a4f94-11c1-4d28-8aac-4d38141829a9">

Updated the assistant header to be a bit more compact, and use a new tab
bar icon style. The existing tab bar icons will be updated in a later
PR.

<img width="646" alt="CleanShot 2023-07-10 at 10 26 30@2x"
src="https://github.com/zed-industries/zed/assets/1714999/3ef9a053-59fa-4d34-9b76-3bb2701acb33">

Updated the remaining token indicator to have 3 steps:
<img width="662" alt="CleanShot 2023-07-10 at 10 29 51@2x"
src="https://github.com/zed-industries/zed/assets/1714999/13d31545-5b00-427c-b7da-b4dfeac037d6">

Updated role labels, added a hover state to make it more clear these are
interactive
<img width="984" alt="CleanShot 2023-07-10 at 10 32 28@2x"
src="https://github.com/zed-industries/zed/assets/1714999/24748495-dde4-4ee9-98f1-6a082f0c1d4d">


Release Notes:

- Improved the UI of some elements in the Assistant panel.