Move away from columns of icons towards the "changed" info dot we used
for files.

Secondary actions for chat/notes still show up (if you're lucky) on
hover.

Co-Authored-By: Marshall <marshall@zed.dev>



Release Notes:

- Improved design of collab panel

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Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>