Merge branch 'hide-todo-collapsed-sidebar' into 'master'

Hide the Todo button in the collapsed issuable sidebar.

## What does this MR do?

Fixes a bug where the "Add Todo" button was displayed in the collapsed sidebar.

## Are there points in the code the reviewer needs to double check?

This shouldn't remove any other elements from the collapsed sidebar.

## What are the relevant issue numbers?

None

## Screenshots (if relevant)

Before:

![Screen_Shot_2016-06-15_at_11.41.46_AM](/uploads/27a1ace58cf8c44ad96accd3d0b32f11/Screen_Shot_2016-06-15_at_11.41.46_AM.png)

After:

![Screen_Shot_2016-06-15_at_11.40.53_AM](/uploads/91a8585680fc482682d0efafdba4d599/Screen_Shot_2016-06-15_at_11.40.53_AM.png)

See merge request !4686

app/assets/stylesheets/pages/issuable.scss

@@ -251,11 +251,16 @@
       padding-bottom: 0;
       margin-bottom: 10px;
     }
+
+    .issuable-header-btn {
+      display: none;
+    }
   }
 
   .issuable-header-btn {
     background: $gray-normal;
     border: 1px solid $border-gray-normal;
+    
     &:hover {
       background: $gray-dark;
       border: 1px solid $border-gray-dark;