PLT-4343 Fixes for mobile main menu (#4148)

* Fixed mobile app link in the main menu to be displayed on mobile browsers

* Fixed doubled up dividers in mobile menu

* Added scrolling to mobile main menu

webapp/sass/layout/_headers.scss

@@ -221,6 +221,10 @@
     .divider {
         border-top: 1px solid transparent;
         margin: 0.5em 0;
+
+        & + .divider {
+            display: none;
+        }
     }
 
     .team__header {
@@ -302,10 +306,6 @@
             .admin-navbar-dropdown__icon {
                 fill: $white;
             }
-
-            .divider + .divider {
-                display: none;
-            }
         }
 
         .user__picture {

webapp/sass/layout/_sidebar-menu.scss

@@ -5,6 +5,7 @@
     border-right: $border-gray;
     display: none;
     height: 100%;
+    overflow: auto;
     padding: 0 0 2em;
     position: absolute;
     right: 0;