Remove 1px whitespace between nav tabs and underline when form present

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/stylesheets/framework/nav.scss

@@ -100,6 +100,7 @@
 
     > form {
       display: inline-block;
+      margin-top: -1px;
     }
 
     .icon-label {
@@ -110,7 +111,7 @@
       height: 34px;
       display: inline-block;
       position: relative;
-      top: 1px;
+      top: 2px;
       margin-right: $gl-padding-top;
 
       /* Medium devices (desktops, 992px and up) */