Feature/site from item set

[Daniel-KM]

In item resource form, it is possible to assign sites, but not in item set resource form. So this feature is added here, and subsequent implications (display sites in item set view, define a user setting for item sets.

There is a last point that is not managed: to add or not to add a site option to assign item sets to it, like it exists for items. For now, the process use the same key "assign_new_items" to manage process. In fact, there are three possibilities:

• use one column in table site, so rename assign_new_items to and assign_new_resources.
• let existing column and change its meaning (like in this pr).
• let existing column in table site and use a site setting to manage the option for item set.
• use two columns in table site: assign_new_items and assign_new_item_sets.

So the point is should we manage items and item sets options separately or not? The same for acl.