Small changes to css to clean up code

web/sass-files/sass/partials/_post.scss

@@ -130,13 +130,6 @@ body.ios {
 		font-size: 2em;
 		margin: 0 0 0.3em;
 	}
-
-	.invisible {
-		visibility: hidden;
-	}
-	.visible {
-		visibility: visible;
-	}
 }
 
 #post-list {

web/sass-files/sass/partials/_sidebar--left.scss

@@ -6,6 +6,7 @@
     border-right: $border-gray;
     padding: 0 0 2em 0;
     background: #fafafa;
+    z-index: 5;
     &.sidebar--padded {
         padding-top: 44px;
     }