Bug 12937 [Follow-up] Deleting suggestions should ask for a confirmation

Bug 12937 [Follow-up] Deleting suggestions should ask for a confirmation

This minor follow-up revises the language a bit: Adding missing question
marks and using the more standard prompt "Please select at least one..."

To test, follow the previous test plan and confirm that the prompts are
improved:

- go on the suggestion management page
- don't select any suggestion and try to delete (check the checkbox and
  submit the form). Verify that you get an alert.
- select 1 suggestion and delete it, verify you get a confirmation
  message
- select 1+ suggestions and delete it, verify you get a confirmation
  message

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>