Bug 27123: (follow-up) Add note to warn about removing all messages

Bug 27123: (follow-up) Add note to warn about removing all messages

This patch adds a note to the batch patron modification form in order to
warn the user about removing multiple messages at once.

To test:

- Apply the patch and go to Tools -> Batch patron modification
- Enter or select multiple patrons and submit them for modification
- On the page previewing the list patrons for modification, scroll down
  to the form.
- Find the field for "Message" and confirm that there is a note next to
  the checkbox.

Signed-off-by: George Williams <george@nekls.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>