This PR makes a first pass at styling the notification panel.

#### Signed out

<img width="381" alt="Screenshot 2023-12-20 at 11 41 25 AM"
src="https://github.com/zed-industries/zed/assets/1486634/f045fa17-4ebc-437f-a25b-d7695d47f18b">

#### No notifications

<img width="380" alt="Screenshot 2023-12-20 at 11 44 23 AM"
src="https://github.com/zed-industries/zed/assets/1486634/3a7543f2-8cd8-4788-8059-d5663f5f6b4c">

#### Notifications

<img width="386" alt="Screenshot 2023-12-20 at 1 27 08 PM"
src="https://github.com/zed-industries/zed/assets/1486634/13b81722-c47a-4c06-b37d-e6515cbfdb9d">

Release Notes:

- N/A