Make sidebar scrollable. It prevents content overflow for small screens

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

app/assets/stylesheets/generic/sidebar.scss

@@ -2,6 +2,8 @@
   .sidebar-wrapper {
     position: fixed;
     top: 0;
+    bottom: 0;
+    overflow-y: scroll;
     left: 0;
     height: 100%;
     transition-duration: .3s;
@@ -21,8 +23,9 @@
 }
 .nav-sidebar {
+  margin-top: 29 + $header-height;
+  margin-bottom: 50px;
   transition-duration: .3s;
-  margin: 0;
   list-style: none;
   overflow: hidden;
@@ -88,9 +91,6 @@
     width: $sidebar_width;
     .nav-sidebar {
-      margin-top: 29px;
-      position: fixed;
-      top: $header-height;
       width: $sidebar_width;
     }
@@ -114,9 +114,6 @@
     width: $sidebar_collapsed_width;
     .nav-sidebar {
-      margin-top: 29px;
-      position: fixed;
-      top: $header-height;
       width: $sidebar_collapsed_width;
       li a {
@@ -178,7 +175,7 @@
 }
 .sidebar-user {
-  position: absolute;
+  position: fixed;
   bottom: 0;
   width: $sidebar_width;
   padding: 10px;

app/assets/stylesheets/themes/gitlab-theme.scss

@@ -38,6 +38,7 @@
       border-right: 1px solid $color-darker;
       .sidebar-user {
+        background: $color-darker;
         color: $color-light;
         &:hover {