LP#1616980 webstaff: protect "magic statuses" when editing copies

LP#1616980 webstaff: protect "magic statuses" when editing copies

This patch fixes a bug where the web staff volume/copy editor
could be used to put items in or out of the following status:

1 - Checked out
3 - Lost
6 - In transit
8 - On holds shelf
16 - Long Overdue
18 - Canceled Transit

To test
-------
1. Choose to "Edit Items" on an Available copy.
2. Status field is fully editable. Click to open the dropdown
3. Click "Checked out"
4. Click "Save & Exit"
5. Result is that the item status changes to 'checked out',
   which should not happen.
6. Choose to "Edit Items" on an Checked out copy.
7. Status field is fully editable. Click to open the dropdown
8. Click "Available"
9. Click "Save & Exit"
10. Result is that the item status changes to 'available'.
11. Apply the patch and repeat steps 1-10. This time, the magic
    status are marked as disabled in the copy status selector.
    If the item is already in one of the magic statuses, if the
    user attempts to change the status, the interface will appear
    to allow it, but the change will be silently ignored.

Signed-off-by: Jason Etheridge <jason@equinoxinitiative.org>
Signed-off-by: Andrea Neiman <abneiman@equinoxinitiative.org>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>