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/_content.scss

@@ -20,7 +20,6 @@
             // stays on the top and don't get hidden by the
             // search's input block
             float: right;
-            margin-right: -18px;
             padding-right: 0px !important;
             position: relative;
             z-index: 6;

webapp/sass/layout/_headers.scss

@@ -426,6 +426,7 @@
 .app__body {
 
     .channel-header__links {
+        display: inline-block;
         font-family: 'Open Sans', sans-serif;
         font-size: 22px;
         height: 30px;

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;