Minor bug and UI fixes (#6729)

* PLT-6797 - Code block language tag not selectable

* PLT-6771 - Aligning search box with RHS

* PLT-6827 - Fixing link color in error bar

* PLT-6241 - Adding edge detection

webapp/sass/layout/_markdown.scss

@@ -92,6 +92,7 @@ h6 {
 .post-code__language {
     @include opacity(0);
     @include transition(opacity, .6s);
+    @include unselectable;
     background: #21586d;
     border-radius: 0 0 0 2px;
     color: $white;