Simplify admin side menu layout

We can move the button to toggle the menu to the main content, and
everything will look the same way while we'll need one div less.

app/assets/stylesheets/admin.scss

@@ -293,24 +293,16 @@ $sidebar-active: #f4fcd0;
 
   .side-menu-and-admin-content {
     display: flex;
-
-    @include breakpoint(small only) {
-      flex-direction: column;
-    }
   }
 
   .side-menu {
     flex: 25%;
     min-width: 250px;
 
-    > :last-child {
-      height: 100%;
-
     .admin-sidebar {
       height: 100%;
     }
   }
-  }
 
   .admin-content {
     flex: 75%;

app/views/layouts/admin.html.erb

@@ -20,17 +20,15 @@
           <%= render "layouts/admin_header" %>
 
           <div class="side-menu-and-admin-content no-margin-top">
-            <div class="side-menu">
+            <div id="side_menu" class="side-menu hide-for-small-only">
+              <%= side_menu %>
+            </div>
+
+            <div class="admin-content">
               <div class="show-for-small-only">
                 <button type="button" class="button hollow expanded" data-toggle="offCanvas"><%= t("admin.menu.admin") %></button>
               </div>
 
-              <div id="side_menu" class="hide-for-small-only">
-                <%= side_menu %>
-              </div>
-            </div>
-
-            <div class="admin-content">
               <%= render "layouts/flash" %>
               <%= render "layouts/officing_booth" if controller.class.parent == Officing && session[:booth_id].present? %>
               <%= yield %>