Make dropdown selectables

This makes dropdown to look according the dropdowns on the edit form. This options add a tick mark next to the selected option.

app/views/projects/protected_branches/_create_protected_branch.html.haml

@@ -25,6 +25,7 @@
             .js-allowed-to-merge-container
               = dropdown_tag('Select',
                              options: { toggle_class: 'js-allowed-to-merge wide',
+                             dropdown_class: 'dropdown-menu-selectable',
                              data: { field_name: 'protected_branch[merge_access_levels_attributes][0][access_level]', input_id: 'merge_access_levels_attributes' }})
         .form-group
           %label.col-md-2.text-right{ for: 'push_access_levels_attributes' }
@@ -33,6 +34,7 @@
             .js-allowed-to-push-container
               = dropdown_tag('Select',
                              options: { toggle_class: 'js-allowed-to-push wide',
+                             dropdown_class: 'dropdown-menu-selectable',
                              data: { field_name: 'protected_branch[push_access_levels_attributes][0][access_level]', input_id: 'push_access_levels_attributes' }})
 
     .panel-footer