Fix EZP-22628: Children containing ezuser can not be trashed

Link: https://jira.ez.no/browse/EZP-22628

## Description
Object containing ezuser attribute can not be sent to trash. It has been the case for a while (and for 10 years for the user class).
If you delete the parent of one of these object (and the parent don't have an ezuser field) ezpublish will try to send it to trash and this will trigger side effects. A quick example is to delete a user_group containing users.

This patch modifies slightly the behavior:
- The "move to trash" checkbox will always be present. If a subtree item contains an ezuser attribute, it will be deactivated and an explanation message will be present (only if needed).
- Children will also be checked to decide if a tree can be sent to trash. If one wants to trash a tree containing ezusers, they will need to go delete them first.

## Screenshot
![delete_user](https://cloud.githubusercontent.com/assets/4035241/2732234/bffd03d4-c637-11e3-92f9-18dba6417a7f.png)


## Tests
Manual sanity check on content deletion.

## TODO
- [ ] Translations ?