Use a different mixin for Notebook container

Use a simpler mixin for Notebook's "inner" container: we don't need
all the stuff in `panel`, `card-frame` suffices. This eliminates the
need to override a vertical rhythm rule.

src/styles/annotator/notebook.scss

@@ -26,7 +26,7 @@
 .Notebook__inner {
   position: relative;
   box-sizing: border-box;
-  @include molecules.panel;
+  @include molecules.card-frame;
   padding: 0;
   width: 100%;
   height: 100%;
@@ -37,7 +37,6 @@
   width: 100%;
   height: 100%;
   border: none;
-  margin-top: 0 !important; // disables the margin-top set by vertical-rhythm mixin
 }
 
 .Notebook__close-button {