message: add aria label
diff: <ExpandButton
  data-analytics-name={{if this.isCollapsed 'Expand' 'Collapse'}}
  local-class='Toggle'
  + aria-label={{if this.isCollapsed 'Expand side nav' 'Collapse side nav'}}
  @onClick={{action this.toggle}}
  @isCollapsed={{this.isCollapsed}}
  @icon={{if this.isCollapsed 'chevron-right' 'chevron-left'}}
  