Bug 7784 - Improve clarity of batch modification operations

Bug 7784 - Improve clarity of batch modification operations

- Improve clarity of hint about checkboxes
- Inicate that leaving fields blank will make no change
- Add a hint when checking checkboxes stating that the field will
  be deleted.
- Block deletion of required subfields
- Add "required" style to required subfields (matching additem.pl)

Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Works as described

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>