allow removal of alchemy_roles in UI

The clear buttons have disappeared due to a change in
alchemy_cms: AlchemyCMS/alchemy_cms#3252

Since we want to allow admin users to remove roles from users,
we need to add the clear buttons here.

Author: robinboening

app/views/alchemy/admin/users/_fields.html.erb

@@ -16,7 +16,8 @@
     input_html: {
       multiple: true,
       is: 'alchemy-select',
-      class: 'alchemy_selectbox'
+      class: 'alchemy_selectbox',
+      data: { allow_clear: true }
     },
     as: 'select' %>
 <% end %>