Bug 15884 - Vendor contract deletion warning is incorrectly styled

Bug 15884 - Vendor contract deletion warning is incorrectly styled

When deleting a contract in  Acquisitions -> Vendor -> Contracts, the
deletion confirmation message is not styled with the standard "dialog
alert" <div>. This patch modifies the confirmation markup to match the
standard.

To test, apply the patch and go to Acquisitions -> [ a vendor] ->
Contracts

- Try deleting a contract. The confirmation box should have the standard
  "dialog alert" class.
- Test the 'confirm' and 'cancel' actions and make sure both complete
  correctly.

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>