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/utils/_functions.scss

@@ -21,3 +21,13 @@
     -moz-osx-font-smoothing: grayscale;
     transform: translate(0, 0);
 }
+
+@mixin unselectable {
+    -webkit-user-select: none; /* Chrome/Safari */
+    -moz-user-select: none; /* Firefox */
+    -ms-user-select: none; /* IE10+ */
+
+    /* Rules below not implemented in browsers yet */
+    -o-user-select: none;
+    user-select: none;
+}