Implement new modal designs

They aren't yet used anywhere, but this will let us move on to implementing specific modal interactions from the new designs.

I made the design decision of making this new Modal component always be controlled by an explicit open state, which was inspired by some work I did with Jetpack Compose recently, where I saw that this makes state management and the behavior of components so much more obvious.

src/index.css

@@ -150,7 +150,9 @@ body {
 html,
 body,
 #root {
-  height: 100%;
+  /* We use !important here to override vaul drawers, which have a side effect
+  of setting height: auto; on the body element and messing up our layouts */
+  height: 100% !important;
 }
 
 #root {