This PR reworks the rendering of the collab panel to use `gpui::list`,
so that we don't render any items that are not visible on the screen.

In the process we also fixed some bugs in the channel list:

- Fixed the context menu for channels not deploying when activated via
keyboard
- Fixed drag and drop for channels
- Made it so when navigating the collab panel via keyboard the list only
scrolls enough to reveal the next item when navigating to an item that
is currently off-screen

Release Notes:

- N/A