help: Prevent scrolling when cursor is on body.

When you hover above the navbar, the cursor focuses on the page
body and scrolls the entire page, breaking the positioning of the
fixed sidebar and page content.

We disable scrolling on the body but allow the sidebar and Markdown
content page elements to be scrolled to fix this bug.

static/js/portico/help.js

@@ -207,4 +207,5 @@ function scrollToHash(container) {
         update_page(html_map, path, container);
     });
 
+    $('body').addClass('noscroll');
 }());

static/styles/portico-signin.scss

@@ -185,6 +185,8 @@ html {
 
 .header {
     background-color: #fff;
+    position: fixed;
+    width: 100%;
 }
 
 .header .top-links a,

static/styles/portico.scss

@@ -2,6 +2,11 @@ body {
     background-color: #fafafa;
 }
 
+.noscroll {
+    overflow: hidden;
+    position: fixed;
+}
+
 .container-fluid {
     padding: 0px;
     min-height: 100%;
@@ -117,6 +122,7 @@ body {
     position: fixed;
     width: 100vw;
     height: calc(100vh - 59px);
+    margin-top: 59px;
     left: 0px;
 
     box-shadow: 0px -20px 50px rgba(0, 0, 0, 0.04);
@@ -1926,6 +1932,7 @@ input.new-organization-button {
     .app.help {
         position: absolute;
         height: calc(100vh - 40px);
+        margin-top: 39px;
     }
 
     .app.help .sidebar {