avoid cutting off menus that hang outside threads

Nice trick here -- by using the hover state we hopefully ensure that
it's hard to activate a menu that gets cut off by the overflow.

h/css/common.scss

@@ -573,6 +573,10 @@ blockquote {
     padding-left: $thread-padding;
     margin-left: -$thread-padding;
 
+    &:hover {
+      overflow: visible;
+    }
+
     & > * {
       border-left: 1px dotted $grayLight;
       position: relative;