[Design] [Cosmo] Add/Remove Collections buttons

[Mimi Yin]

Here's a quick mock-up

Travis, can we re-use the button backgrounds we use elsewhere?

Ideally, the buttons would line up with the width of the mini-cal  
horizontal divider lines.

They vertical gap between the buttons and the Go-to-date widget  
should be the same as the vertical gap between the Go-to-date widget  
and the mini-cal navigation bar.

It would also be nice if we could get rid of the left and right  
borders for the mini-cal navigation buttons. Otherwise, the sidebar  
starts to feel overly compartmentalized.

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