Merge branch 'nav-scroll-bug' into 'master'

Side nav scrolls if content height taller than screen

## What does this MR do?
Header logo is now fixed at top; put back scrolling functionality in side nav

## Screenshots (if relevant)
<img src="/uploads/e1d1b60c296c39c61d7fe7da22b6c52d/scroll.gif" width="600px">

See merge request !4513

app/assets/stylesheets/framework/sidebar.scss

@@ -38,6 +38,11 @@
   .header-logo {
     height: $header-height;
     padding: 8px 26px;
+    width: $sidebar_width;
+    position: fixed;
+    z-index: 999;
+    overflow: hidden;
+    transition-duration: .3s;
 
     &:hover {
       background-color: #eee;
@@ -73,7 +78,8 @@
 
 
 .nav-sidebar {
-  margin: 22px 0;
+  margin-top: 22 + $header-height;
+  margin-bottom: 116px;
   transition-duration: .3s;
   list-style: none;
   overflow: hidden;
@@ -166,6 +172,7 @@
 
     .header-logo {
       width: 0;
+      padding: 8px 0;
 
       a {
         padding-left: ($sidebar_collapsed_width - 36) / 2;