Admin feature: Ability to change the owner of an item

[amyehodge]

Add an admin option on the item view page to change the owner of an item.

When selected, admin sees a box for entering in the sunet ID of the new owner. The admin should be shown the name of the person to verify this is correct.

The mockup below shows the error state when the SUNet is not found.
@astridu I think we may need more mockups of what's happening for this.

There are some other associated tasks that go along with this:

• notify the new owner that they now own an item
• notify the collection manager that the ownership has changed and that they may now have a new user in their collection
• if the new owner was not already a depositor in that collection, add them as a depositor